On a Florida Notice of Commencement (NOC) the "interest in property" refers to the specific type of ownership you, as the owner, has in the property. Most homeowners will be classified as a "fee-simple;" meaning full ownership.
The reason this designation is required, is that a Notice of Commencement can also be signed and filed by a lessee, who would designate their interest in the property as a "leasehold interest." This is further supported by the fact that the NOC form requires that the fee-simple owner be listed if the owner listed has an interest "other than fee-simple." If you are the full owner of the property, your interest would be "fee-simple."
Here's an article that you may find helpful: Florida Notice of Commencement Checklist & Lien Risk Guide For Owners