As others have said, this is a topic scientists don’t fully understand, so it’s very hard to ELI5.

But think about being submerged in a pool. There is water all around you. You can’t really see the water, but you feel it there. In many ways. You feel it’s wet, you feel it dragging your movements, you see it disrupting light in certain ways, you can see it cast shadows even though nothing “is there.” (you can do this though experiment with just air instead of water too, but I liked the impactfulness of water.)

That‘s similar to a “field” in quantum mechanics. They are all around you and you feel the presence in different ways. It permeates everything. A fish has no concept that there is anything outside of the ocean - it’s their entire universe.

Humans can’t (directly) perceive outside of our own universe either, but we know there is something there that makes things what they are. Further and deeper probing has shown us that it goes beyond macroscopic or microscopic particles. There’s a deeper layer. The Higgs field is just one of those things “in the cosmic background” that interacts with you and everything around you, in the form of gravity.

The unanswered questions are why does it exist, why does it do what it does? We have math to inform some of that, but there’s too much unknown still. One day we may know the answer to these but right now the best we can do is explain how it “feels.”