Books set market limits by bet type (moneylines higher, props lower) and adjust individual player limits based on betting history. If a book cuts your max bet from $2,000 to $50, you've been "limited" — usually because you're winning or showing sharp patterns.
Advanced concepts
Bet limit
The maximum amount a book will accept on a given bet — can be reduced for winning bettors.
Example
Pinnacle: $500K game limit on NFL sides. DraftKings: $5 limit for a flagged account on player props.