TL;DRAn operator approval lets one address move every item you own in that collection, including items you receive later. A single-item approval covers one token id and clears when the item moves. Close operator approvals you no longer use with setApprovalForAll(operator, false).
Two kinds of NFT permission
ERC-721 has two approvals. approve(to, tokenId) names one address for one item; it is cleared automatically when that item is transferred. setApprovalForAll(operator, true) names an operator for the whole collection; it stays until you switch it off. ERC-1155 collections only have the second kind.
Marketplaces usually ask for the operator kind, so that one approval lets them settle any sale you make in that collection.
What Nullo shows
Operator approvals appear as Collection operator with the amount All items, and the review hint reminds you that it covers future items too. Single-item approvals appear with their token id and are re-read with getApproved, so an item that has moved on shows as closed.
Closing them
For an operator the revoke call is setApprovalForAll(operator, false) on the collection contract. For a single item it is approve(0x0000…0000, tokenId). Both are shown in full before your wallet opens, and the row is read again once the transaction lands.
Try it on your own address
Read your open permissions in a minute.