Pressing Tab will automate combat for one turn, causing your character to attack the nearest hostile monster with the weapon you are currently wielding. Reaching weapons will take advantage of their reach and ranged weapons will fire whatever ammo you've got quivered, but weapons that can be thrown without a launcher cannot be automated; you must fire them manually. If no target is within range, it will cause you to instead move toward the nearest monster.
Although physical combat can occasionally require complex decision making, most fights in Crawl are very straightforward. Along with providing a convenient means of speeding through harmless opponents, Tabbing also helps you avoid certain dangers. Attacking by attempting to move into an enemy can be dangerous when fighting blinking or trampling opponents, as you may suddenly find yourself wasting turns on unintentional movement while your enemies get free attacks. Additionally, using the tab key will stop the fight when the player gets too low on HP (50% by default), potentially prompting the player to reconsider the fight.
Tabbing through combat does have its downsides, however. Tabbing characters will move into attack range without concern for giving the opponent a free first attack. When in range of multiple opponents of varying strengths, the AI may choose to attack a weak target while neglecting actual threats. Movement through Tabbing does not take terrain into account, resulting in very poor choices in areas where traps and shallow water are plentiful. It doesn't take too many clumsy blunders to teach a player when to avoid the Tab key.