For "For intX = 1 to 100 step 5", what is the purpose of the step clause?

Answer 1

I believe it means the count goes like this: 1, 6, 11,16, 21, 26, etc. This is the increment is 5.

Answer 2

The 'step x' function tells BASIC how much of a 'jump' to take to the next X...in your example, BASIC would add 5 so the output would be your starting number + 5.
You can also have it count in the negative direction by adding the minus mark "-" to the step value.