What it is
The free-text section at the bottom of the LES where the pay system announces what changed: pays starting and stopping, debts established, allotment changes, and notices. It is the closest thing your pay account has to a changelog.
What goes wrong
Read the remarks every single month. Debts, stopped entitlements, and coming changes are announced here before they surprise you. Most pay problems that 'came out of nowhere' were in the remarks a month earlier.