"lol" is an acronym, which is a word formed from the initial letters of a longer name or phrases, in this case, "lol"stands for "laugh out loud".
It's an internet slang term that people use to indicate that they think something is really fun.
The Boston Public Library has a list of "netiquette" suggestions, or ways to interact on the Internet in a friendly way.
It's important to stay +ve and friendly when you're talking with people, even over email!