Substance Painter has two viewports: a 3D and a 2D view. Both views allow to paint on your 3D mesh. Both views also offer the possibility to visualize your texture under a lighting.
At the bottom of the viewports you can see a gray line that sometimes turns green. It's a computation bar that indicate the progress of something the engine of Painter is computing.
You can cycle between different layouts for the views via the following toolbar icons (and their associated keyboard shortcuts):