App Inventor Control Blocks
This page of documentation details how to use each control block in App Inventor. What we're most interested in is the "if" block and the "for each number from" block.

Blockly.games
Look familiar? This is the activity from week 1! If you need a refresher on how if blocks work, check out the Maze tutorial again.

For Loop
This is a wikipedia article and its pretty long. Just read the synopsis at the top and look at the diagram off to the side to get an idea of how the loop works.

Casey's Explanation of a For Loop

A loop is a control flow statement that makes something repeat itself over and over until you tell it to stop. The loop requires three things: the number you start at, the number you stop at, and how to get to that stopping number. Consider this loop:

For each number from 1 to 10 by 1:
    do: Show the number
End for

Those are English words, but they're a little wonky. They read "Loop a number from 1 to 10, show the number" Meaning we'd see the numbers 1, 2, 3, 4, 5, etc. because we're increasing number by 1 each time we loop.

How would you make a loop count from 1 to 1000, counting by 3's instead of 1's?

For each number from 1 to 1000 by 3:
  do: Show the number
End for

This says "Loop a number from 1 to 1000, showing the number, increasing it by 3 at the end of each loop". We'd see 1, 4, 7, 10, 13, 16, etc. because we're increasing number by 3 this time.

As with most things in computer programming, a lot of this will make more sense when you do it for reals! See the lesson videos for details.