Can someone tell me why a 20-character key makes WPA Personal more secure? That really confuses me.
The WPA2-Personal password is run through an algorithm that is used to scramble the internet traffic being broadcast in radio waves. This is a process known as encryption. Anyone who knows that password can run the scrambled data through the encryption algorithm in reverse to de-scramble it. Although a human might be able to try a new password once every few seconds, a computer could guess possibly millions of passwords per second. More characters in your key makes it less likely that a computer could guess your password in any reasonable amount of time.
If someone manages to guess your password, either manually or with the help of a computer, they might be able to monitor things you do online for as long as they are in range of your Wi-Fi connection. They may steal passwords or bank details, or insert malicious scripts into websites you visit.