Newbie gains happen when people first start exercising, and their bodies respond quickly. Their muscles are considered “hyper-responsive”.
Beginners often see rapid progress because their bodies aren't accustomed to training stimuli, leading to quick strength and size gains.
Basically, the muscles get stronger and bigger because they're not used to the exercise.
On the other hand, if someone has been in the gym for 3+ years, their bodies take longer to grow muscle because their bodies are used to stimulating their muscles. So in comparison, beginners gain muscle faster.
Now, the basics of fitness still need to be in play for optimal results. Fundamentals such as progressive overload, correct calorie intake, and proper recovery are still a necessity.
Don’t think newbie gains are just magic.
Usually around 1-2 years after being in the gym consistently, a beginner will start to plateau in progress. So if you are a beginner, make sure to take advantage of those newbie gains while you can!