"Steering" something means controlling which direction it moves in. For example, you steer a car when you drive.
We also use the word "steer" to talk about controlling a conversation. You steer a conversation when there's something that you want to talk about, or something that you want to avoid talking about.
When people bring up politics, I usually try to steer the conversation back to something a little less controversial.
If you'd like to return to a topic that you were talking about earlier, you "steer the conversation back" to that topic.