What does "let down" mean?

The food of this restaurant let me down

I’ve just received a rejection letter from the university I’m longing to go. It really lets me down….....

When i was a child,i often was told don’t let your parents down,but you know ,i always let my parents down at that time ,all because my studying,i like playing with my classmater and always forgot back home on time,so my mother very angry every time,some time ,i can not get the score which my father’s hope,let my father down,actually ,every one want to try them best to avoid let someone down.nowadays,i found that my parents becoming older day by day,what i want to do just don’t let them down again

This job salary truely let me down.