The memory64 proposal adds support for linear WebAssembly memories with size larger than 2^32 bits. It provides no new instructions, but instead extends the existing instructions to allow 64-bit indexes for memories and tables.
This feature allows WebAssembly applications to address and use more than 4GB of memory if the system allows for it (64-bit architecture and sufficient available memory). It furthermore allows 64-bit indices to be used throughout WebAssembly (e.g. for table indices) for consistency.
Explainers: https://github.com/WebAssembly/memory64/blob/main/proposals/memory64/Overview.md