That is when he cums
Finishing is important. It means to come but it could also mean to finish a sex session "cum on her face" or get off somehow.
In terms of blow jobs, finishing for me means coming in her mouth and her swallowing. I don't bust on her face unless she asks me, which some girls enjoy. Some guys I know only like to come on a girls face. I know girls don't believe this but some girls like to feel dirty in a naughty but nice way.
You are under 18, so I'd practice this stuff a little bit because it is important in relationships to please a guy sexually. He will be so much better to you and adore you if you can get him to finish often.
He has an orgasm. He ejaculated! He came.
For him to cum. It's just a nice way of saying it