Often in English, two words can look and sound very similar, but have completely different meanings. Distinguishing between these words can be challenging. Take a close look at these words and notice that one has an “s” on the end. This changes the meaning. Beside is a preposition. It means near, next to, or at the side of. I will stand beside you through good times or bad. She stood beside the fence gate.
Besides is an adverb. It means in addition to, apart from, or except. It was the most popular book ever written, besides the Bible. Everyone besides Anna wanted to go swimming.
In conversation, native speakers can understand what you’re meaning, even if you choose the incorrect word. However, in writing it is important to think carefully about which word you choose.