What does it mean when sperm can't penetrate the eggs

Normally a sperm can easily penetrate the egg, but if another sperm has already entered, or the sperm and/or egg is defective. Ignore the other answer because it merely restates your question. So a sperm can always penetrate an egg unless it is too late or either it or the egg is defected.
Hope this helps!

It means that there the egg already penetrated by another sperm