Beside is a preposition of location. It means “next to:”
- There’s a nightclub beside our house, so it’s often really noisy.
- I have a small table beside my bed.
- The calculator is beside the notebook.
Besides means “in addition” or “apart from”:
- What did you do on your vacation besides sleep?
- Besides Sarah and Joanna, there were no women at the meeting.
- I didn’t eat anything at the restaurant because I wasn’t hungry – and besides, I don’t like Italian food.