“You know what they say: where there's a will, there's a way.”
You are raising three young children as a single parent while also working full time. You're talking to a coworker who says that it must be hard for you. It is, but you're able to survive. You say this in response.
You know what they say: where there's a will, there's a way.
When you want to repeat a famous proverb or saying, introduce it by saying "You know what they say:" For example:
You know what they say: if it's not broke, don't fix it!
You know what they say: absence makes the heart grow fonder.
It's common just to say the first part of the famous saying. Everyone knows the rest of it, so it's not necessary to say it all. For example:
You know what they say: When in Rome...
The rest of the saying is "When in Rome, do as the Romans."
This is a famous saying. It means that you can accomplish anything if you work hard.
In this case, "will" means a strong motivation to accomplish something. So you can understand the phrase to mean "If you have a strong motivation to accomplish something, there's a way to accomplish it."