A masculine first-person pronoun meaning 'I' or 'me'; colloquial and can sound rough or arrogant.
俺はもう行くよ。
I'm going now. (masculine, casual)
俺の名前は田中だ。
My name is Tanaka. (casual male speech)
彼は怒ると『俺がやった』と大声で言った。
When he got angry he shouted, 'I did it.'