What is a changelist in IntelliJ IDEA?
Changelists, when used with Git, is a way to split your index (i.e. the files you have added to git) into logical filesets. See Using Git Locally with IntelliJ IDEA
Those changelists can represent whatever you want (a task, a fix, a merge etc), but they are not git commits.
You can commit one or several changelists.
When you add a file to the Git Index, IntelliJ IDEA adds your file to the default changelist, named... "default".