Metadata freezing is the act of updating an NFT's metadata in a way that cannot be changed. The exact mechanics of this depend on how the contract was written, but here's two examples.

1. If an NFT contract's metadata comes from onchain data only, then if there was flag variable which could be set to disable updates, setting that flag would make it no longer possible to change the metadata. This would be a freeze.
2. If an NFT contract's metadata comes from a URL which is immutable, such as an IPFS link, then if there's a way to disable the modification of the base URL, that would also be a freeze. This is because the IPFS link contents cannot be changed, and the NFT cannot be directed to another URL anymore.

In both cases, once the flag is set, there is no possible way to change the metadata.