In professional sporting leagues, rookies are known as first year players, while vets (short for veterans) are experienced and have established a name for themselves in the league. Here, by calling himself a rookie and a vet, Drake takes the best of both words. Because Drake was still really young (27) at the time he wrote this song, he calls himself a rook to the rap game; but at the same time, he talks about how he's already a veteran, due to the fact he'd already put his name out there as a top rapper in the industry in a much shorter time than most other artists did.