They would represent any generic two-digit number.
After enough play for this to make sense, the next level of abstraction would be to draw what you just did. A circle representing tens digit (the white towers) and a square, the gray box (ones digit). Try drawing and filling in the squares with the tens digit and the circles with the ones digit until it makes sense. Then see what happens when you don't fill in the blanks. The same process happens regardless of the tens digit and the ones digit.
The final leg of the journey to abstract land would be to represent the two-digit number, yx, where y is the tens digit and x is the ones digit. You've been doing the math all along with objects and drawings. The expressions are simply a way to represent what you've already been doing algebraically. If reading the final line of expressions makes your head swim, try going through the process with manips and by drawing first. DON'T JUST READ! PLAY!
Going abstract, let zyx be the three-digit number in which z is the hundreds digit, y is the tens digit, and x is the ones digit. You can see what happens below each picture.
Going abstract, let azyx be the four-digit number in which a is the thousands digit, z is the hundreds digit, y is the tens digit, and x is the ones digit. You can see what happens below each picture.