A slur is used to show that two or more notes are smoothly joined together. In a song, slurred notes would be sung to the same syllable of text and without taking a breath; in a violin piece, slurred notes would be played on a single stroke of the bow. A piano player would try to imitate the slurred effect by holding each melody note until the next one has begun, so that there would be no break between them.
To slur notes, select them and then click on the Slur tool. The Unslur tool can be used to remove a slur from selected notes. Slur curves are adjustable - to see the control points, hold down the Shift key and click in the slur curve using the arrow cursor (the Shift requirement allows you to select a curve even if it overlaps notes). When the boxes appear you can drag them and see the curve change.
When you insert or delete a note within a slurred group, the slur will remain. But if you delete the first note of the slur, the slur will be deleted as well; it is attached to the first note. You can reslur the remaining notes, if desired, with the slur tool.
Remember that the notes you originally selected are the ones that are slurred: though you can drag one of the slur's end points so that it appears to include other notes, they are not really slurred. This could create problems for lyric alignment because regardless of how the slur looks, Counterpointer will consider as truly slurred only the notes that were selected when you pressed the slur button. If you want to expand a slur group it's best to unslur it and reslur. On the other hand, there may be occasions where a slur that doesn't affect lyrics could come in handy.