Definition
When IL2CPP converts IL to C++, it strips the .NET metadata out of the assemblies and writes it to one file under il2cpp_data/Metadata. The runtime loads it to resolve types, support reflection, and read string literals. The runtime pairs this file with the native binary to resolve classes, methods, fields, and those strings.
Relevance
The native binary has no .NET type names of its own. global-metadata.dat is the table the IL2CPP runtime uses to resolve types, run reflection, and read string literals, which is why the original class and method names are still in an IL2CPP build.
Example
At startup the IL2CPP runtime loads global-metadata.dat, resolves PlayerWallet.AddCoins from it, and reads string literals from the same file. The machine code for AddCoins stays in the native binary.