The timeline view is a summary of all your daily activity. Light green bars are for regular walking steps (step counts and minutes of walking), and dark green are for brisk walking. The red lines are inactive times, with total consecutive minutes of sedentary time marked in red to the left.
You can scroll up and down to see your stats for other times of the day. You can scroll to different days using the arrows left and right of the date at the top.
To see a summary of your weekly stats, click on the “Weekly” tab in the upper right.