The word slough has two completely different meanings that aren't related to each other:

1. A swampy or marshy area, or a shallow, muddy pond. You might find a slough at the edge of a wetland where water pools without draining away completely. Birds like herons often hunt for fish in sloughs, and frogs thrive in these muddy spots. The Slough of Despond in The Pilgrim's Progress is a bog that represents feelings of discouragement and hopelessness.
2. To shed or cast off something, the way a snake sloughs off its old skin as a new one grows underneath. Trees slough their bark, and your own skin constantly sloughs off dead cells (some of the dust in your home is made of these). People also use the word figuratively: you might slough off bad habits or slough off criticism that doesn't matter.