Freezing your metatdata will permanently lock and store your files on the Blockchain. This means that your item will be safely stored and will never be lost or go missing & cant be edited or changed.
On Opensea when you freeze your metadata the following information will be locked: The item name, the media file type, the description and the properties, levels & stats. All of this you added when adding a listing to the site. Unlockable content will not be frozen as that is only available to buyers of your NFT.
There is an Ethereum fee to freeze your metadata. Also be warned: Once frozen your content CANNOT BE EDITED OR REMOVED. Make sure you get it right. Once done it will tell you in the details tab that the metadata has been permanently locked.