It’s hard to tell, but that looks like an old scab for that opening. Someone took a spare 2x4 and added to that opening to add more strength to that area. However it looks like it was poorly done. Regardless, the 2x4 is compromised and the red-wood 2x4 isn’t fully nailed into the footer. I am not a carpenter, but if I were you, I’d build a proper frame for whatever you’re trying to install in that opening. Found this good example of what I’m talking about: [rogueengineer.com/installing-a-window-in-a-blank-…]