The Lord is the Portion of mine inheritance. God had said to Aaron, when he gave him no special inheritance in Canaan, "I am thy Part and thine Inherit-ante among the children of Israel" (Numbers 18:20).
David claims the same privilege. God is his "Portion," and he needs no other. And of my cup. A man's "cup" is, in Scripture, his lot or condition in life (Psalms 11:6; Psalms 23:5)—that which is given him to drink.
David will have God only for his cup. Thou maintainest my lot; i.e. thou makest it firm and sure (comp. Psalms 30:6, "In my prosperity I said, I shall never be moved").