What does the title long shadows of warriors mean?

It means that:

There is an old saying that says "Old sins cast long shadows" which means the bad things you have done will follow you through life. so in the warrior cat book 5 of the power of three. The three kits parents are Leafpool and crowfeather wich is the old sin(she broke the warrior code, she is a medicine cat) and the effects of that will catch up with her and him and change the future of the clans because the kits weren't supposed to be born yet there is a prophecy about them.