The various tissues of the body involved in its' movement have different inherent rates of adaptation. This means that as physical stress is introduced to the system, certain tissues adjust to the stress (gain strength) faster than others.
For example, skeletal muscle tissue is astoundingly efficient at adapting to physical stressors. Muscular capacity increases dramatically within the initial 3-5 months of the untrained "popping their cherry". This is well known in gym-rat circles; a dude benching 135 on day one could be throwing up reps of 225 by day 120. But, then what happens?
Plateau. Dun dun dunnnnnnnn.
What is a plateau? The mythology of plateaus is vast and diverse; some say they're due to a need for more food intake, others claim the muscles are getting 'bored' and require a change-up in routine. Gym junkies are a creative breed. But, what are they, really?
They're your body expressing astounding wisdom, and limiting itself accordingly. A person is actually capable of lifting far more weight than their body generally allows them to. This is a protective program installed to ensure fragile hardware isn't corrupted, but a program that can be overridden nonetheless. Case in point: the classic mom-lifting-car-off-baby scenario. This exertion gate can be deactivated if the situation calls for it, but generally speaking the program runs like a background app.
Essentially, when a plateau is reached, it represents the body saying, "Stop. We're on the brink of disaster, and I refuse to voluntarily let that happen." Instead of listening to this blunt bodily advice, the new trainee's passion and excitement from seeing their initial gains motivates them to "push through" their plateau. As my left shoulder, mid-back, and right hip will attest: this is a bad idea.
Plateaus happen because while sure, one's muscles have quickly gained physical capacity, their tendons, ligaments, and bones are all begging for a chance to catch up. As a general model, what muscle can adapt to within six months takes ligaments and tendons about twice as long to comfortably handle. Bones are on a much longer time scale, somewhere in the ballpark of 5-7 years, but this doesn't usually cause an issue in the average trainee. You only ever hear about bones snapping with competitors in strongman/Olympic-lifting competitions or contact-sport related scenarios.
What this means to you:
The new initiate on the path of physical training needs to recognize the absolute need for patience and consistency instead of zealousness and intermittency. The hardest part of being hooked on working out is learning to slow your roll; this "PUSH YOURSELF!!!" mentality encouraged by magazines and gurus is dangerous when taken too far. There are myriad horror stories of religious crossfitters, for example, significantly hurting themselves because they thought they knew more about their body than their body does. Before one can begin barbell squats, for example, they need to develop the hip flexibility required to adopt a full ass-to-floor squat position, without any added weight, and without needing to stick their arms out in front of them for balance. Essentially, if you can't do a perfect overhead squat with a broomstick, you have no place loading a bar with excessive weight on your traps. . . unless of course you'd rather roll the dice to buy some temporary gains. Weak, inflexible hips knocked me off my path for an entire summer 2 years ago; I could barely walk; whole lotta good my passionate "PUSH YOURSELF!!!" attitude did me then.
My experience and research leads me to recommend that any initiate of the training path keep this Principle of Varying Adaptation in mind. A squad of soldiers can only effectively move as fast as its' slowest member, and the path of physical training is not a 3-month gym membership. That's vanity. Vanity's great to a degree; I mean, it's the primary motivation for getting people on the path. However, the path continues for one's entire life, and vanity is not powerful enough to persevere through its entirety. The path is a part of Life. It's Life-Style. It's part of our human-ness, for when one is not in a state of being physically trained they are not normal; they're sick. Patience, self-control, and consistency, are the only ways to properly navigate the journey. When a plateau is reached, begin to focus your efforts on building the strength of the other tissues involved in your movements. This translates into performing stability, flexibility, and balance-required exercises to build up the slower-adapting tissues involved; eventually they will catch up. I've personally taken a liking to yoga for this purpose; after hurting my hip a few years ago doing squats, yoga fixed it. When the complete matrix of tissues involved have been allowed to fully adapt, the plateau will cease to exist in the bodymind. Until then, don't be an idiot like I was; we've got the rest of our lives to progress.