According to the Apparel Glossary from ApparelnBags.com, "When slub yarn is spun it forms intermittent lumps in the thread due to irregular twisting." The lumps (slubs) appear thicker than the surrounding thread when it's knit into fabric. It creates a burnout-like appearance without the transparency of a burnout T, and with easier printability.
The lumps and variance in the fabric give a vintage, weathered look to a cotton T-shirt. The lumps in the slub yarn also add an element of texture to a cotton T-shirt.
(Images: Stellar Textiles Etsy)
Deluxe.com says slub T-shirts are breathable and light, not clinging close to the body.
To explore mixing specialty fabrics like slub with SFX inks, check out this video.