Definition
This idiom advises against criticizing others for faults or weaknesses that one possesses themselves.
He is always criticizing others for being late, but he himself is often late; it's a classic case of 'people who live in glass houses shouldn't throw stones.'
Before you accuse her of mistakes, you should remember that 'people who live in glass houses shouldn't throw stones', as you've made the same error.
Their boss often criticizes employees for not working hard enough, but he himself rarely works overtime, which is a perfect example of 'people who live in glass houses shouldn't throw stones'.