The emoji black square button belongs to the group emoji symbols, and is therefore a symbol of something. Usually, the name of the emoji will reveal what it is a symbol for.
Here you'll find more info about the emoji 🔲. The main group that the emoji black square button belongs to is Symbols, a group that it shares with another 304 similar emojis. A lot of emojis does also exist in different variations, for example different skin colors, the emoji black square button does however not have any known variations.
If you want to copy the emoji in order to use it in for example an email or a chat, all you have to do is click the button "Click here to copy emoji" that is listed below the emoji. The emoji black square button will then be placed in your clipboard, and you can easily paste it the way you normally paste copied text - for example by pressing ctrl+v (or cmd+v if you are using a Mac) if you are using a regular computor. Please note though that this function is disabled on mobile phones due to techincal issues, there you'll have to hold your finger towards the emoji and copy it the regular way once the dialogue opens.
Below you will also find the HEX code for the emoji together with a prepared example coded in HTML that is ready for you to use directly in a generic HTML document.
Use in HTML: