Shoutbox

At the moment I'm doing databases and SQL  I understand most of what can and can't be done. But one thing really confuses me

How to use indexing.

I understandish how it works by creating unique records with more than just a primary key.

Like you can't use a compisite key to have the same person on the same date going to the same event  (if that makes sense)  as that person could aslo uniquely be sent to a different event on the same day.

But I just can't work out how indexing helps the situation.