Here’s some info to decode what this means:
Bend Allowance is a compensation for what happens to the material as it’s stretched and formed from a flat pattern into a proper bend. Calculating this value will let you know how much material will be left on either side of the bend once it’s formed.
To see what this would look like as a flat pattern, here’s another diagram:
By knowing how much material the bend is going to consume, we can work out exactly where to place the bend.