An Adjustment is a manual change Race Space has made to your account, either adding funds or removing them. You will see it in your Payment History, inside your Finances section.
The usual reason
Most adjustments happen because you have chosen to add funds to your Race Space account by bank transfer, so Race Space has credited your account to match.
Other reasons
There are various other reasons an adjustment might appear, though they are rare.
If you see an adjustment and are not sure what it relates to, email [email protected] and Race Space will explain it.