"Random" things are things that:

• you don't expect
• you can't predict
• you can't explain

"Random" is used a lot in mathematics, science, computer programming, etc. to talk about events or numbers which can't be predicted.

These days, young people use the word "random" quite a lot. For example:

A: Hey, guess who called me? Jake from high school.

B: Really? That's so random!