What is meant by the expression “to teach one’s grandmother to suck eggs?”
It doesn’t seem to mean anything.
It doesn’t seem to make sense.
Who the heck sucks eggs?
Is it a remnant of our dim prehistoric past, when nothing turned a frown upside down like enjoying a fresh raw egg stolen from a nest, while out pursuing mastodons?
The traditional explanation is that it refers to the practice of “blowing eggs”, which is where you put a tiny pinhole in one end of a raw egg, to let air in, and a bigger pinhole in the other end, to let egg-stuff out, and then you put your mouth on the tiny pinhole and blow. The air pressure will force the egg-stuff out.
Then you decorate the egg, and have it to keep forever, isn’t that wonderful. 
No clue as to how this came to mean “don’t teach your elders how to do stuff that they already knew about before you were even born, buster…” However, it’s possible that before the era of compulsive arts ‘n’ crafts, it may have been a useful survival skill, to know how to do this. For one thing, it’s a good way to get the edible egg part out, leaving you with a nice empty unbroken eggshell to store something in, like, I dunno, beeswax or something.
Kindly give us some kind of warning before using the words “grandmother” and “suck” in the same sentence, lest we audibly scream at work.
In the old days, man Grandmothers were missing their teeth, and would eat raw eggs for sustenance (easy to eat without teeth). They knew how to suck eggs, and therefore, as DDG mentioned, it would be useless to try to teach your Grandma how to suck eggs.
[WAG Quotient: 75%]