LOB_DATA is your varchar(max), nvarchar(max), varbinary(max), XML and a few other data types. They have special storage.

Typically you want to filter for 'in row data' when reindexing because reindexing affects only the in row data pages (reorg can compress lob data too, but that's not fragmentation in the traditional sense). If you don't, you can end up rebuilding the same index up to 3 times, which is a waste of time.