〜が (but): but (between clauses); excuse me, but…

After a clause, が means "but": it sets up a contrast, or softly leads into what you want to say.

Polite clause + が: 高いですが、おいしいです. It is also a polite opener before a request or question: すみませんが…. Casual speech uses けど. This is not the subject marker が (私が行きます); the clue is that a full clause comes before it.