Casual expression said when leaving (similar to 'I'm off' or 'see you later'); implies you'll go somewhere now and return later.
ちょっとコンビニに行ってくるね。
I'm going to the convenience store for a bit (I'll be back).
もう遅いから、先に行ってくるよ。
It's getting late, so I'm off ahead of you.
出かけるときに『行ってくる』と言って玄関を出た。
He said 'I'm off' and left through the front door.
Verb meaning 'to go (somewhere) and then come back'; indicates departing now with the expectation of returning afterward.
彼は書類を持って会社に行ってくると言って出かけた。
He said he'd go to the office with the documents and come back, then left.
明日は大阪に行ってくる予定です。午後には戻ります。
I'm going to Osaka tomorrow and will come back; I'll be back in the afternoon.
子どもが学校へ行ってくるのを待っていた。
I was waiting for the child to go to school and come back.