As Welsh says - Cold Drawn Seamless steel tube - it's mechanically drawn over some dies to remove the welded seam and refine the grain structure to strengthen the steel.
The basic rule of thumb is CDS is twice the strength of ERW (your general steel tube for most things), and T45 is twice as strong again*
(*assuming decent materials, not always the case, a lot of 'CDS' sold by the unscrupulous for car/rollcage use is CFS NBK - basically it's been heated and normalised/softened in an oven to make it easy to bend for hydraulic work and as a result it's nowhere near as strong as mechanical spec tube, and doesn't actually meet the required MSA specs for competition cages. That's why I always supply mill certificates with rollcages for the tube.)
________________________________________- Bespoke rollcages/additions/adjustments. Half cages right up to complete custom spaceframes - MSA/FIA spec, CDS, ROPT, T45, etc - PM me
Team had to rebuild my gearbox. Twice.