A casual contraction of 'sou ka' used to express understanding or realization ("oh, I see"). Common in spoken, informal contexts.
「明日は休みだよ」「そっか、じゃあゆっくり休むね。」
"Tomorrow's a holiday." "Oh, I see — I'll take it easy then."
彼が来られないって。」「そっか、連絡ありがとう。」
"He can't come." "I see—thanks for letting me know."
そっか。そういうことだったんだね。
Oh, I see. So that's how it was.