This is the 5th most frequent Greek word.
που
"Που" means "that," "which," or "where," depending on context.
In this sentence, 'που' is used as a relative pronoun to introduce a subordinate clause—specifically, to describe 'the dog'.
Ο σκύλος που είδα ήταν πολύ φιλικός.
The dog that I saw was very friendly.
Here, 'που' connects clauses by introducing an explanatory or clarifying additional statement.
Μου είπε ότι ήρθαν, που σημαίνει πως θα τους δούμε σύντομα.
He told me they arrived, which means we'll see them soon.
In this situation, 'που' functions within a structure to introduce a subordinate clause, further specifying 'the one'.