This is the 7395th most frequent Turkish word.
geldin
"You came."
The word "geldin" here is the past tense second-person singular form of the verb "gelmek," meaning "to come." It denotes a completed action of arrival.
Buraya geldin ve her şeyi değiştirdin.
You came here and changed everything.
In this sentence, "geldin" represents the past tense, indicating that the action of coming occurred earlier.
Neden bu kadar geç geldin?
Why did you come so late?
The use of "geldin" again is to indicate the past arrival action. It connects the fact of an early arrival with the subsequent positive sentiment.