Okay, here's an ELI5 explanation of Lagrangian Grassmannian:
Sometimes, we have a big space that we can break up into smaller spaces that all share some important qualities. We call these smaller spaces "subspaces". Like how a house is a big space, but you can break it up into the living room, the kitchen, and the bedrooms, which are all smaller subspaces.
Now, sometimes we're interested in finding subspaces that have a certain special property that we care about. In math, one special type of subspace is called a "Lagrangian subspace". These subspaces are special because they have the same "size" or "dimension" as their complement, which is like two pieces of a puzzle that fit together perfectly. We call this property "maximal isotropic".
The problem is, it can be really hard to find all the Lagrangian subspaces in a big space. This is where Lagrangian Grassmannian comes in. It's like a map that helps us find all the Lagrangian subspaces in a space. It's called a "Grassmannian" because it's named after a mathematician called Grassmann who studied subspaces. And it's called "Lagrangian" because it helps us find Lagrangian subspaces.
The Lagrangian Grassmannian is basically a giant set of all the possible Lagrangian subspaces in a space. It's like a big book with all the different ways you can cut up a big space into Lagrangian subspaces. But instead of being a book, it's a math concept that helps us understand the structure of spaces better.
So, to sum up: the Lagrangian Grassmannian is a way to find all the special subspaces in a space that have a really important property called "maximal isotropic". It's like a map that shows us all the different ways we can slice up a space into these special subspaces.