a unique, mappable key that allows the use of combining symbols to create a nonstandard character
a table of all the default bindings can be found here
If you copy the default binding file, which on Pop!_OS is at /usr/share/X11/locale/en_US.UTF-8/Compose, to ~/.Xcompose, you can then add your own custom bindings, I have the following addition, with more probably on the way
<Multi_key> <asterisk> <asterisk> :"★" U2605 # BLACK STAR