Time trials are a unique sort of of bike race where you are racing against the clock. You don't have any other sort of competitors on the course. What you're being measured by is the amount of time it takes for you to complete a set distance.
Time trials are usually known to be incredibly hard because you have to push yourself as fast as you can go for a long period of time. They can range from a couple of miles to 30 or 40 miles. It's very important to be able to meter your efforts. You don't want to blow up. At the same time, you want to go as hard as you can as fast as you can for the entire time on course.
You don't have the benefit of drafting any other competitors. You have to do all the work yourself. There's far less strategy involved in time trials. The main goal is to go as fast as you can as hard as you can without blowing up.
And that's the basic description of a time trial.