With the Credits subscription (€ 35 per month) you get 30 credits every month to spend freely at partner sports locations. How many credits a class costs depends on the sport and location. On average, this lets you work out about 4 times a month.
With the Unlimited subscription (€ 50 per month) you don't need credits and can work out unlimited across the entire Workit offering. Not sure which subscription suits you? You can switch every month, so you're never locked in.