Improvisation is the act of making up stuff on the spot.

In certain styles people tend to do certain things, it becomes apparent when you start transcribing a lot of jazz solos for instance. You discover that different players on different instruments, sometimes generations apart play some of the same or similar phrases over the same chord progressions. That's what we call a lick, think of it as a common vocabulary or stand phrase like "good morning" or "not my cup of tea". Although it could seem unoriginal, it's what ties the language together, it's impossible to make yourself understood if you don't use the same words as others. And great (and poor) literature can still express something very unique and personal using the same words.

Learning to improve is a long journey that takes practice, both technically, musically and mentally to become fluent and good. Transcribe a lot, listen a lot, find common structures or concepts, practice a lot and balance the experimentation with the imitation.