This is a paraphrase of Psalms 16:5:
יְהוָה מְנָת-חֶלְקִי וְכוֹסִי , אַתָּה תּוֹמִיךְ גּוֹרָלִי
It means that God is my "portion". Just as every man has his "portion" in life, the same way that a person is given a portion of wine in his cup at a meal, the true servant of God has God as his portion in his life. It is what his focus in life is. (God is also called the "חלק" of Jacob in Jer. 10:16: "לֹא-כְאֵלֶּה חֵלֶק יַעֲקֹב", as well as Ps. 119:57: "חֶלְקִי יְהוָה אָמַרְתִּי".)
The phrase "מנת כוסי" is used here in the sense of "my portion" and as interchangeable with "חלקי" (and also in Ps. 11:6: "וְרוּחַ זִלְעָפוֹת--מְנָת כּוֹסָם"). "מנת" is the possessive form of "מנה", which means "portion" just like "חלק" in this context. According to Radak, "כוסי" also means "portion" here and these words are put together as if one is possessive of the other even though they are separate nouns, just like "אדמת עפר" (Daniel 12:2). According to others, though, "מנת כוסי" just means "מנה הנופל בכוסי", i.e. the portion that is in my cup - my share - in this life.