When you affix something, you stick it on to something else. You might affix a stamp to a letter or affix colorful stickers to your notebook.
The verb affix often refers to physically attaching something — like when you affix a label onto a folder — but it can be used more broadly to describe adding on something. You might affix your signature to a contract. If you like grammar, you might already know that the noun form of affix refers to letters or words that can be tacked onto other base words to alter their meaning. For example, the prefix un- is an affix.