Some IDA commands such as selecting a portion of file to output or specifying a segment to move need an anchor.
To drop the anchor, you can either use the Alt-L key or the Shift-<arrow> combination, which is more convenient. You can also drop the anchor with the mouse by simply clicking and dragging it.
After you've dropped the anchor, you can navigate freely using arrows, etc. Any command that uses the anchor, raises it.
The anchored range is displayed with another color.
When you exit from IDA, the anchor value is lost.