In other words home/draw/away.
This is the short form of the 3-way bet, where 1 means home team win, X - draw/tie, and 2 - away team victory. It is mostly used in football betting, but you can also find it in other sports where the tie game is possible.
1X2 is also the most popular type of bet but is it the most efficient from the tipster's point of view, though? There's no single answer to this. Let's not forget about the most important detail, it is a 3-way bet, which means there are three different possible outcomes of the event (win/tie/loss). Coming from that, we only have a roughly 33% chance of winning the bet, instead of 50% like in the case of two-way handicaps or Over/Under bets.