what is the difference between viewer and traverse in access level

You can find some more information about this kind of access under FolderSharingInfo, e.g., under FolderMetadata.sharing_info as returned by /2/files/get_metadata:

"traverse_only Boolean Specifies that the folder can only be traversed and the user can only see a limited subset of the contents of this folder because they don't have read access to this folder. They do, however, have access to some sub folder"

 

For example, if a user should have access to a nested shared folder, but not the rest of the contents of the parent shared folder, they may have "traverse" rights to the parent shared folder so that they can "traverse" to the nested shared folder in the hierarchy without unnecessarily being able to access the rest of the parent shared folder contents.