Sorry I can't think what they are called! You put your outdoor bike on it and peddle on the road to nowhere. Like a treadmill for your bike 
I'm wondering if they are worth getting? Now my children are on holiday I'm not able to do my weekly bike ride as no child care and I go too far for the youngest to come with me.
I'm reaching maximum density and need to keep my exercise up over the summer. I'll already be down 3-4 miles on my daily walk due to lack of school run and I fear I'll look like the Michelin man by September 