It means that when a batter
for the team trying to score runs allows themselves to be hit by a ball
thrown by an opposing pitcher.
When a player is hit, he gets a free pass to first, and therefore has a chance to score a run.
Players who are not good hitters are often urged to take one for the team late in a close game, even though this may mean getting hit in the ribs with a ball thrown at around 80 miles per hour.
Major league expert at this skill: Craig Biggio