What does line 18 do? (Regarding Social network pet project)

just tell me what does this line do,like what is this attribute ‘data-username’
Screen Shot 2020-11-17 at 1.37.43 AM

Hello,

The data-* (in this case data-username) attribute is a standard HTML attribute that can be used to store custom data that can be accessed with JavaScript. It isn’t specific to React. You can read more about it here: HTML data-* Attributes