An expression meaning to be hurt or injured (to have sustained a wound or injury). It is the past/state form of 'to get injured'.
昨日、自転車で転んで足にけがをした。
Yesterday I fell off my bicycle and hurt my leg.
彼は試合中にけがをしたため、交代させられた。
He was injured during the match and was substituted.
けがをしたときはまず消毒して、必要なら病院に行ってください。
If you are injured, first disinfect the wound and go to the hospital if necessary.