The word only has several related meanings:

When describing how much of something exists, only means nothing more than that amount. If you have only three dollars, that's all the money you've got. If only five students raise their hands, no one else does. It emphasizes the smallness or limitation: “I studied for only ten minutes” means you didn't study very long.

Only also means exclusively or solely. Your only friend at a new school is your one friend there. An only child has no brothers or sisters. If you can solve a puzzle only by working backward, that's the single way to do it.

When only appears at the start of a sentence, it can introduce something that happened just recently: “Only yesterday I couldn't ride a bike, and now I can!” This usage emphasizes how new or surprising something is.

The placement of only in a sentence can change the meaning. “I only ate carrots” can suggest you ate them but didn't eat anything else. “I ate only carrots” means carrots were all you ate. Pay attention to where you put it.