meaning - What does "I have straight A's." mean?

In this video of Hillary Clinton, at 15s, the child said "I have straight A's.".

What does "straight A's" mean?

Answer

In the U.S., examinations are traditionally graded by letter of the alphabet, commonly A-F, with A being the highest pass grade.

A 'Straight A' student is one that has achieved an A Grade across all subjects taken