Tags are how you label content or people on social media. You tag people by mentioning their username (@username), and you tag topics using hashtags (#topic). Both increase discoverability and engagement.
Tagging people notifies them and connects your content to theirs—their followers might discover you through the tag. Tag brands when you feature their products (they might share), tag collaborators to give credit, tag friends to include them in memories. Don't spam-tag irrelevant people for attention. That's annoying and will get you unfollowed or blocked. Tag purposefully and relevantly.
Product tags on Instagram and Facebook let you tag items in posts, making them shoppable. When someone clicks the tag, they can purchase directly. This is huge for e-commerce businesses. Location tags (geotags) mark where content was created, helping local discoverability. Tag your business location in posts so people can find you.