You can know that it exists and replaces an ordinary piano with the help of computer technology - when you press a key, the computer simply emits a sound recorded for this key. For everyday life, this knowledge is quite enough)
If you are thinking about buying, then you need to first of all decide on your desires.
1) Do you need a speaker in the piano or you will it is enough to hear music from the computer / headphones.
2) Do you need sounds only from the piano, or do you still want to record some other things there. Or what kind of instruments should be recorded in memory.
3) How many keys do you need. In a regular piano there are 88 of them, in an electronic one there may be less. For complex melodies you need 88-85, for some synthesized music for your own entertainment, even less may be enough.
4) Key weight. On a regular piano, they are quite heavy due to the mechanism. On an electronic piano, they can be simple (like a button on a computer), or they can be half-weighted and full-weighted - heavy, like a real piano. It is difficult to change from a simple piano to a real piano.
Well, in accordance with your goals, select based on these parameters.