we have the phrasel verb let sb down, which means disappoint, so let somebody down means: to disappoint someone by failing to do what you agreed to do or were expected to do. For example:
When I was sent to prison, I really felt I had let my parents down.
however we have other uses of this verb, like let something down that has diffrent meaning.( from cambridge online dictionary).
if you want more information visit this very good dictionary:
Cambridge Dictionaries Online - Cambridge University Press