A "tack rag" is a lint-free cloth that has a very mild adhesive pre-applied to it, used to remove residual sanding dust and other contaminants from a surface prior to spraying paint. They are commonly used in autobody paint prep, among other things, but primarily on cars where the dust can REALLY stand out and downright ruin a paintjob. They're available from paint shops and maybe some auto supplies, especially if they have a wide selection of paint and prep materials.
Technically speaking, you CAN 'tack' off the surface of your rockets with just a damp paper towel, though that's NOT REALLY a tack rag and it doesn't take off virtually all the dust or contaminants as effectively as a true tack rag does... but for most rockets in most conditions, about 99% of the time it's all that's required, and definitely cheaper and easier than messing with 'real' tack rags...
Good luck! OL JR