All sin leads to death, of course. However, Jesus' ransom sacrifice can save us from sin and death. (Romans 5:12; 6:23) Thus, a “sin that leads to death” would not be covered by Christ’s ransom. Here is the meaning of what John meant in a nutshell.
1 John 5:16-17 Updated American Standard Version (UASV) 16 If anyone sees his brother sinning a sin not leading to death, he shall ask, and God will for him give life, to those who commit sin not leading to death. There is a sin leading to death; I do not say that he should make request for this. 17 All unrighteousness is sin, and there is a sin not leading to death.