It's a method of applying a printed design to a garment. This involves a transfer process which uses heat and pressure to take the design from paper to the surface of the fabric - giving you total freedom over the design and colour of your kit!
Sublimation is great for a number of reasons: it is super versatile meaning you can have a design as wacky as you like, in any possible combination of colours. Sublimation also prints everything at once, including names, numbers and any logos meaning that you can add as many of these as you like without affecting the cost. Because the logos are transferred as part of the design, it keeps the garment light which is always a bonus and enables you to play to your best.