What does it mean a sin leading to death and…

What does it mean a sin leading to death and a sin not leading to death?

“If any man see his brother sin a sin which is not unto death, he shall ask, and he shall give him life for them that sin not unto death. There is a sin unto death: I do not say that he shall pray for it. All unrighteousness is sin: and there is a sin not unto death.” – 1 John 5:16-17.