The word indoors is an adverb. Indoors means (1) within a building.
The following articles and web pages elaborate on the word indoors.
Cycling Trainers
A cycling trainer is designed for cycling indoors when weather conditions are not optimal. Learn more and find links to cycling trainers and stationary bicycle manufacturers.
https://www.BicycleAccessories.us/trainers.htm
Ellipticals
Elliptical machines combine the motion of stair climbers with that of treadmills, and are a popular way to get an aerobic workout indoors. Elliptical trainers provide a low-impact workout.
https://www.Ellipticals.us/
Exercise Bikes
An exercise or stationary bike is an effective way to get an aerobic workout indoors. It is ideal for when the weather is bad, and it can be used by cyclists for training during the off-season.
https://www.ExerciseBikes.us/
The following digital image illustrates the word indoors.