A script is bound to a Google Sheets, Docs, Slides, or Forms file if it was created from that document rather than as a standalone script. The file that a bound script is attached to is called a "container." 

Bound scripts generally behave like standalone scripts except that they do not appear in Google Drive, they cannot be detached from the file they are bound to, and they gain a few special privileges over the parent file.

Note that scripts can also be bound to Google Sites, but these scripts are almost always deployed as web apps. 

Scripts bound to Google Sheets, Docs, Slides, or Forms can also become web apps, although this is uncommon.