ROT-X allows you to encrypt plain text. "X" refers to a variable number of letters to shift. The most famous is ROT13, in which each letter is shifted halfway through the alphabet. And there is a ROT13 font that makes it easy to see and use.
ROT-X is simple to decrypt, because to undo the first rotate (ROT is short for rotate), you should rotate again by 26 - X. So a ROT13 will reverse a ROT13, a ROT6 will reverse a ROT20, etc.
This page could also be called ROT-N, where N (or n) is any number of times to shift the letters. But the page would be pronounced as "rotten", so "rot-x" sounds better.
Enter a number in the ROT- field, enter a phrase in the Phrase field, and then click the button to encrypt the text.