Spinning refers to a high-cadence style of pedalling, typically 90-100+ rpm. It became a real buzzword in cycling after the high-rpm exploits of a certain Texan in the Tour de France, but of course his performances were down to other factors rather than just pedalling technique!
However, there’s still a school of thought that, by spinning at higher cadences, cycling efficiency is improved and muscular strain and load reduced. From a tri perspective, this led to people suggesting that, by spinning on the bike, your legs are fresher for the run. Although there’s some research that backs this up, it’s far from conclusive.
Also, in the real world of racing rather than the lab, there are a number of top triathletes who fare extremely well with a slower grinding style on the bike. For example, Caroline Steffen typically rides in the 70-80rpm range.
Like many aspects of performance, optimal cadence is highly individualised and a number of studies have shown that a rider’s self-selected cadence is often their most efficient. Certainly experiment with higher-cadence spinning, it might work for you, but it’s not the cycling panacea for all it’s sometimes made out to be.