Phloem is one of the main types of vascular tissue in a plant. Phloem is made up of live cells, unlike xylem. Phloem tissue carries sugars all around the plant. Phloem tissue is flexible. Sugars leave the phloem into surrounding cells by diffusion, providing surrounding cells with nutrition.
Phloem makes up the inner bark of trees. If you cut the phloem all around the trunk (called ring-barking the tree), the tree will die since the roots will no longer be able to get 'fed' by the sugars produced in the leaves by photosynthesis.