The lesson this proverb teaches is that people whose own conduct is open to criticism should not criticise the conduct of other’s who may retaliate by accusing their accusers.
‘When are you going to return that book I lent you over a month ago? It’s downright dishonest to keep it so long.’
‘What about the one you borrowed from me early last year?’
‘I’m sorry. I’d forgotten all about that one.’
‘People who live in glass houses shouldn’t throw stones.’