Paul reminds the Corinthians that it is the gospel that saves them and he tells them again what the gospel is. Then he gives them a short history lesson which ends with what will yet happen.
Possible meanings are 1) Paul wants to remind them by making the gospel known again, "help you remember, brothers, the gospel" or 2) he wants to make sure they understand the gospel, "make clear to you, brothers, the gospel."
Paul is speaking of the Corinthians as if they were a house and the gospel as if it were the foundation on which the house was standing. (See: [[rc://en/ta/man/jit/figs-metaphor]])