For Next repeats block of statements based on the counter variable, counter variable is incremented after each execution by adding Step value. Counter variable is assigned Start_expression value on entry into the loop. Block of statements is executed if the counter variable value is less then or equal to the End_expression for positive Step values. For negative Step values the condition is more then or equal.

Loop with default Step 1

For Var = Start_expression To End_expression

...

Next Var

With Step specified

For Var = Start_expression To End_expression Step Expression

...

Next Var

Wikipedia: Count-controlled loops

Wikipedia: For loop