Meld deals with differences between files as a list of change blocks or more simply changes. Each change is a group of lines which correspond between files. Since these changes are what you're usually interested in, Meld gives you specific tools to navigate between these changes and to edit them. You can find these tools in the Changes menu.
In addition to directly editing text files, Meld gives you tools to move, copy or delete individual differences between files. The bar between two files not only shows you what parts of the two files correspond, but also lets you selectively merge or delete differing changes by clicking the arrow or cross icons next to the start of each change.
The default action is replace. This action replaces the contents of the corresponding change with the current change.
Hold down the Shift key to change the current action to delete. This action deletes the current change.
Hold down the Ctrl key to change the current action to insert. This action inserts the current change above or below (as selected) the corresponding change.