This is a phrase taken from Psalm 23 in the King James version of the Bible, and describes means being given more than enough for ones needs. It is quite often used, in a variety of contexts, to describe having ‘plenty’ of something. The exact verse is addressed by the speaker to God and is:
Thou preparest a table before me in the presence of mine enemies: thou anointest my head with oil; my cup runneth over.
This psalm is also the basis of the well know hymn ‘The Lord is my Shepherd’.