말하는 건

what someone is talking about

→ 말하다
phrase Uncommon #12,783
what someone is talking about 건 is a contraction of 것은, used to turn the preceding verb phrase into a topic. The exact meaning depends on context.
KO: 제가 말하는 건 그 문제가 아니에요.
EN: That’s not what I’m talking about.