path variable with %% and # meaning

[Anis1973]

Hi, I'm a beginner in shell and I have theese two lines of code :
$ echo ${PATH%%:}
$ echo ${PATH#:}
I know that the first line prints the first directory of the PATH variable
and the second one prints the rest of the directories in PATH.
But i don't understand the parts of code inside the curly brackets and how does it work .
can I get help please !?