Sorry, I didn't mean to freak you out or anything!! Guppies are pretty hardy fish and with the two fish you have in there now, you should be able to get through cycling with no problems! The problems that I was talking about were what would happen if you added more fish right away and overstock the tank.

What does cycled mean?
Chris explained cycling pretty well. Fish waste and leftover food creates ammonia, which is basically toxic. The more fish you have, the higher your ammonia levels will be in the beginning. "Cycling" is the process where your "good" bacteria establishes itself in the tank. This bacteria is what naturally breaks the toxins down. This is the natural filtering process.

The main purpose of a mechanical filter is basically to aid this natural process. In every filter, there will be a type of media where the bacteria lives. Different filters use different kinds of media (i.e. a sponge cartridge, bio-balls, etc.) The filter sucks up waste so that the bacteria bed can break it down.

How important is the gravel and why?
The gravel basically provides more places for the good bacteria to live. Think of it like another filter. The only thing is, the gravel is home to good bacteria AND bad bacteria, so it does need to be vacuumed once in a while. It's up to you if you want to add it.

Do I need a specific kind of filter or will just a normal little internal box one do?
It's really up to you. A better filter means less work for you in maintenance, and you can possibly keep some more fish with it.

What is a "small bio-load"?
A bio-load is basically the "load" that your tank has to support. A small bio-load basically means there isn't too much burden on the bio-filter (or your bacteria colony).