I am a sophomore in high school and I need help with matrices. The category selection doesn't have geometry, the course I am taking, and I really need help. My teacher didn't explain the material very well, so I am really confused.

The short answer is to multiply the ith row times the jth column to get the element in the i,j entry of the product. So, multiplying row 1 of A times column 1 of B gives the element in the 1st row and 1st column of C.