For the chief musician

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

set to the Shoshannim

This may refer to a style of music.

Asaph is referring to God as the one who leads and protects Israel.

"you who lead the descendants of Joseph as though they were a flock of sheep"

Here Joseph represents the nation of Israel.

"you who sit on your throne above the cherubim on the ark of the covenant"

"give us light" or "show us the right way to live"