BTTS means both teams to score, while in some bookmaker sites it is highlighted as Goal Goal(GG) or No Goal (NG). What these simply means that it is highly likely that both teams will score at least one goal against each other. This option should be chosen when the teams playing have a poor defense.
The table below shows the predictions for all the possible matches that can finish with btts.
Let’s say for example, Valencia is playing against Sevilla in a Laliga Match. Both teams are a high scoring team and have a very porous defense after scoring. This gap provides a higher chances of goals being scored thus, your bet for the match will be Goal or BTTS.
However, you can as well place your bet on a match that you are probably sure both teams can't score aganist each other. For example, let's say PSG are playing Toulose in a French Ligue 1 match. There are high chances of PSG winning with a cleansheet. Thus your bet will be NG or No Goal on the match.