The red stripe tests technique.  

Once you have earned the green and blue stripe and are roughly ten classes shy of meeting your attendance requirements, you can test for the red stripe.

Remember that the red stripe on your belt means that you have mastered the technique for the rank you are testing for.  Therefore, it is unlikely to be ready for your red stripe soon after you receive a new rank.  The red stripe is usually one of the last stripes earned.

When you test for your red stripe, you will be tested in four areas:

Stances

Blocks

Strikes

Kicks

Kata

Usually, Sensei will find something from the above four areas that you will need to work on.  This will be written down for you.  Keep that paper, practice hard on the areas you need to work on, and bring it with you when you re-test for your red stripe.  If you bring that paper to Sensei when you re-test, you will not have to take the entire test again, but will re-test only in the areas that need correction.

Sensei will also look at your overall performance during class.  Your technique during class has to show that you are at the level for the rank that you are testing for.