# For the chief musician

"This is for the director of music to use in worship,"

# Jeduthun

One of David's chief musicians had this same name. This may refer to him. 

# I will watch what I say

"I will pay attention to the things that I say"

# so that I do not sin with my tongue

"so that I do not speak an offense against Yahweh" 

# muzzle

To "muzzle" means to keep a mouth shut. Here David means that he will not speak while he is with an evil person.