A colorful term used in place of "everywhere", "all around", "all over" or "a plethora"; i.e. a lot.
I went fishing yesterday and nailled bass left and right.
Last night there were hotties in the club left and right.
I was in a highway pile up and was hit left and right.
We had a picnic on the beach and the seagulls were swarming us left and right.