A changelist is a set of local changes that have not yet been committed to the VCS repository.
All changes made to your project files are automatically placed in the active changelist (normally, it's the Default changelist, but you can make any other changelist the active changelist).
Create a new changelist
Set the active changelist
Move files between changelists
Select the files that you want to move to another changelist.
Right-click the selection or click on the toolbar and choose Move to Another Changelist.
In the dialog that opens, select an existing changelist or enter the name for a new changelist.
You can choose to make this changelist the active one, and to track context for it (RubyMine will save the context associated with this changelist, and will restore it when this changelist becomes active).
Put changes in a single file into difference changelists
When you make a change to a file in the editor, click the corresponding change marker in the gutter.
In the toolbar that appears, select the target changelist for the modified code chunk (or create a new changelist):