When your friends are hungry, you cannot serve them from an empty breadbasket, no matter how good your intentions. When they're thirsty, you cannot serve them from an empty pitcher, no matter the effort you put in.
We have to stock ourselves with good things before we are able to give them away, and if we are too exhausted to make bread, we must rest before we can make it to serve our friends. If we are too hungry to serve, we must eat some bread ourselves first.
It's a metaphor for emotional exhaustion. When we wear ourselves down it makes it difficult or impossible to help the people around us, and all the effort and good intentions in the world can't make up for the fact that our vessels are empty. It's hard to not want to serve when our friends are in trouble, but trying to serve from an empty vessel often leaves both of you floundering.
On the contrary, when we care for ourselves and are kind to our body and mind, we are full vessels, and serving the people around us becomes easier. That's why it's important to take time for ourselves once in a while, to refill ourselves with good things.