In British English more especially for older speakers this phrase is implying that you agree. More casual informal and modern ways to say some thing and also meaning you agree are. 1. yes/yeah absolutely. 2. absolutely. 3. totally agree. 4. yes/yeah totally agree. 5. right on. 6. spot on. 7. could not agree more. 8. you can say that again. (depends on previous context could mean agree or disagree) 9. I think so. 10. I would think so (depends on previous context could be agreeing or almost agreeing with an element of doubt)

In what context?

There are about a million ways to say this. Here are a few: Certainly. I'll say. Damxxx Straight. (any expletive) yeah. You can say that again. Ain't that the truth. True dat. Word. Can't argue with that.

It could also have a literal meaning, but it's most frequently used in British English as a way of emphatically agreeing with something someone has said or as a way of implying that they've understated something. Here are some possible equivalents: No doubt. That's definitely true. You can say that again.