A polite expression meaning to be treated to a meal by someone (to have someone pay for your food).
昨夜は先輩にご馳走になって、いいレストランに連れて行ってもらった。
Last night a senior treated me; they took me to a nice restaurant.
今日のランチは部長にご馳走になりました。とてもおいしかったです。
I was treated to lunch by the manager today. It was very tasty.
旅行中、親戚にご馳走になることが多かった。
While traveling, I was often treated by relatives.
あの店ではご馳走になる前にお礼を言っておくと礼儀正しい。
At that place, it's polite to express thanks before being treated.