BTTS (also 'goals galore' or GG/NG) settles Yes if each team scores at least one goal, No otherwise. A 3–1 is Yes; a 2–0 is No. The result of the match is irrelevant, which makes BTTS popular for games between attacking sides with shaky defences — you can be right about the game's character while being wrong about the winner.
Pricing BTTS well means estimating each team's scoring probability rather than who is better overall. Its close cousin is the over/under goals market: BTTS-Yes correlates strongly with Over 2.5, and comparing the two prices occasionally exposes inconsistencies in how a bookmaker has modelled the same fixture.
If the home side scores in 75% of matches and the away side in 60% (roughly independent), BTTS-Yes ≈ 0.75 × 0.60 = 45% — fair odds 2.22. A price of 2.40 would be value; 1.95 would not.