# A psalm of David

Possible meanings are 1) David wrote the psalm or 2) the psalm is about David or 3) the psalm is in the style of David's psalms.

# A maschil

This may refer to a style of music.

# whose transgression is forgiven, whose sin is covered

"who God forgives his transgression and covers his sin" 

# whose sin is covered

"whose sin is ignored" or "whose sin is deliberately forgotten"