They kept the matter
What the rising
From the dead
καὶ τὸν λόγον ἐκράτησαν πρὸς ἑαυτοὺς συνζητοῦντες τί ἐστιν τὸ ἐκ νεκρῶν ἀναστῆναι.
This is another unique passage of Mark, who tended to point out how these 3 trusted disciples, Peter, James, and John, were confused and still did not understand what was happening. Mark said that the 3 apostles were able to keep this matter quiet among themselves (καὶ τὸν λόγον ἐκράτησαν πρὸς ἑαυτοὺς). However, they questioned or discussed this among themselves (συνζητοῦντες) what the rising from the dead (τί ἐστιν τὸ ἐκ νεκρῶν ἀναστῆναι) was all about.