As a pastor I hear this a lot from parents. Parents want their kids to understand communion or baptism and want me, the pastor, to explain it to them.
The ability to explain communion or baptism to a 3rd grader is so important that many people who are moving toward ordination are asked this question in ordination interviews. As though 3rd graders are the only ones who do not understand.
But in all honesty, you cannot explain the sacraments - no one can.
The sacraments are holy habits that Christians participate in. They are the things that shape us as a community.
I cannot being to share with anyone what the sacraments mean because they are not something that I do. The sacraments are a part of who I am. It is like asking me to describe what the color of my eyes mean or the shape of my hands mean. I cannot tell you, but these things help make me who I am.
We are all called to participate in the sacraments, some people heed the call and allow these holy habits to shape them and form them into the people who they are.
Others are busy trying to understand them as though a good working knowledge of the sacraments matters more than participating in them.
I can explain why someone would participate in the sacraments much easier than I can explain all the different meanings of the sacraments - even to a 3rd grader.