Glossary
Overlays
Horses whose price implies a lower chance than your figure says they have.
An overlay is the entire reason to handicap. If your figure gives a horse a 25% chance and the board pays 5/1 (implied ~17%), the math is on your side regardless of whether the horse wins this particular race.
First Over flags overlays separately from RTPP and steam. They don't always agree, and the spots where they do tend to be the most actionable.