0 to 1 is the relative position. For example taking horizontal placement, 0 means left, 1 means right, 0.5 means center, 0.25 is 25% from the left, etc. The other number is simply added to this calculated value. So if it's 100, it means it's whatever the calculated value is plus 100 pixels.
If you don't want something to resize, anchor both sides to the same side. For example, left to left, right to left. This will make them both stick to the left side of the target widget, and prevent resizing.