Concise Bible Dictionary:
Abraham their forefather was believed by the Jews to be in the highest place of happiness, and their writings show that “to be with Abraham” and to be in his bosom were terms they used to express the highest security and happiness. Our Lord therefore used an expression that was well understood by His hearers and needed no explanation (Luke 16:23). It stands in contrast to hell, or hades, and was therefore figurative of heaven.