Ever think what it would be like to just have a faucet with no sink? Or Vise versa? You can’t have one without the other, kind of like xylem and phloem. They are vital for a vascular plants survival, growth and reproduction. Coupled together never leaving one anothers sides they are the heart and lungs, the source and sink, the sun and the moon. Although not always studied together, xylem has been more broadly studied partially because phloem it’s harder to measure. Xylem is involved in the movement of water through the plants from the roots to leaves. Phloem is involved in translocation, which is the movement of food from the stem to growing tissue and storage tissue. One xylem and one phloem make up a vascular bundle (as seen in the picture). 
These vascular bundles run all throughout the plant. Even though most plants use photosynthesis as a source for nutrients there still can be non-photosynthesizing tissues within plants, like the roots and stems. One of the jobs of phloem is to transport food produced by photosynthesis from the leaves to the non photosynthesizing parts. When I talk about source and sink I refer to different parts of the plants and it also has to do with phloem. According to BasicBiology, the sugars sources are the plants organs that produce sugar, such as the leaves. The sugar sinks are the plant organs that consume or store sugar, such as the roots. 
In the picture to the left it shows how the source and the sink work and in what directions they flow. For nutrients to originally get into the phloem it has to go through the companion cell and the casparian strip. There are three ways this can happen: symplastically, apoplastically or transcellular. The final stage is to pass through the casparian strip which is like a filter, filtering out unwanted nutrients. Just as humans have hearts, veins and organs; plants have similar systems in which they use to keep themselves running and alive. Although that being said, not all plants have vascular systems, but they still have ways to transport the necessary amounts of food and nutrients. An easy way to think about the source and sink, is to think about a human eating food. The food in your mouth is the source, and your stomach is the sink. Even though it’s much more complicated than that it’s still a good way to picture it. If your still confused, below is a video by FuseSchool who have a great explanation on xylem and phloem and the transportation in plants.
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