What Is Encryption
"Encryption", in a broad sense, is to record information in a way that cannot be understood by people who are not supposed to.
At it's earliest a probable and decent method of encrypting information would be to simply write it down, as most people could not read. A form of encryption against those who could read, would involve simply reorganising the letters in a way that the recipient of the information also knew of. For example, Julius Caesar would move each letter 4 places forward in the alphabet, A would become D and C would become F.
Modern day methods are much more complex, "cracking" information encrypted with AES-256 by trying every password, assuming you'd find the correct one half way through, would take 2^255 attempts. That's a LOT of attempts, and although computers are smart and fast, they aren't fast enough to find the password within a human lifetime, or even several. The main threat to encryption now is someone learning the password (you giving it to them, etc.) or there being a flaw in the algorithm that allows someone to find the password much sooner. While the latter do exist for AES, they do not reduce it enough to be considered even remotely "crackable".
Encryption does, of course, have a downside, and that is time. If you've encrypted large files with this website (say, over 20MB), you know that encrypting isn't instantaneous. If your device is single cored and not that fast in the first place, it may have taken a good minute to finish (luckily, most computers are at least multi-cored now). Having to do this with ALL information would slow down computers significantly. This is why not all websites offer secure browsing (HTTPS) and when they do, they may restrict it to just where user information is concerned.