Intervals are simply riding for a specific time at a specific effort. The time is measured by a watch or cyclocomputer and the effort is measured by a power meter, a heart monitor, or rating of perceived effort.

Volumes have been written on the subject, but very generally speaking you go as hard as you can such that you can maintain that effort for the length of the interval, and then repeat. And, also very generally speaking, the shorter the interval, the higher the intensity, the more it works your anaerobic system.

Here's a good one that you could do twice a week: 10 x 1' as hard as you can for the full minute. It's simple, it's short, and you don't need anything more than a watch and a big dose of HTFU.