What does hashtag mean when texting?

Hey there, I'm curious about something that I've been seeing a lot lately in my text messages and social media feeds. Could you explain to me what the hashtag (#) sign means when people use it in their texts? I've noticed that it seems to be used to categorize or group together similar topics or messages, but I'm not entirely sure how it works or what its purpose is. Any insights you could provide would be greatly appreciated!