Jack is five. He lives in a single, locked room with his Ma.
This blurb should tell you what the premise is. Note the present tense used.
In the beginning, for the first hundred pages, all we get is the frame-by-frame, second-by-second relay of things that Jack and Ma do in their time in Room.
How they brush their teeth, what they eat for breakfast, what time they eat lunch, how many teeth Jack has etc. And these things happen over and over again, and we readers are exposed to every second of it.
It gets tiresome.
Linearity without any relief like change of scenes, or change in Point-of-view is laborious. But it is in line with the theme here : holed-up one-roomedness.