Sorry - it was rather idiomatic of me; perhaps I shouldn't have used it in a post aimed at a non-native speaker without explaining it. I'm not sure I'd call it slang exactly, though.
It's a set phrase, you need the "grandmother" bit as well as the "sucking eggs".
You are pretty much right with your interpretation. There's a great article about it here:
World Wide Words: Teaching one’s grandmother to suck eggs
The very erudite Michael Quinion summarises the meaning as "don’t give needless assistance or presume to offer advice to an expert" which I think is preatty good description, but I'd still encourage you to read the whole link, it's fascinating.
I think there is a subtle difference between that and "preaching to the choir" which I always think of as "trying to persuade people who already agree with you" rather than trying to explain something to them. For instance, a post on here trying to persuade people of the benefits of learning English would be preaching to the choir - we all agree it's a fabulous thing already! But a post explaining a certain basic grammar rule wouldn't be teaching my grandmother to suck eggs unless I was replying to one of the very experienced posters who was an English teacher and who, it could be safely presumed, already knew the rule - and probably knew it better than me!