You're thinking of the theoretical casino advantage, which can be below 1% for BJ (favorable rules) and craps (pass or come bets with odds), under 2% (I think) for baccarat, to 5.26% for roulette, and higher for carnival games, keno.

As a simple example, if you bought in for $100 at a $5 BJ table and played the money ONE TIME ONLY (that is, 20 hands), then your expected loss with basic strategy play and average rules is about a 2% loss. (You actually would need a number of such trials to make this work out.)

In reality, if you sit down with $100 at a BJ table, you play many more than 20 hands. Lets say you play 100 hands with it, winning some, losing some. You have had $500 worth of ACTION (100 hands x $5), and at a 2% loss on total action, this equates to an expected loss of $10. You cash out and walk away with $90. The casino made $10 of the $100 originally dropped in the box when you bought in. In this case, the HOLD was 10% ($10 / $100).

Most games are not that favorable, on average, so a 20% or better hold is achievable.