Select the "Fog Of War" token in the hierarchy panel, this token is hidden by default but can be made visible in the token details sub-panel. Once it is visible it will create an opaque layer in the scene hiding everything below it in the hierarchy from players. Only the GM can see through this layer and its opacity can be adjusted for the GM in the token details sub-panel.
To reveal areas use one of the shape tools, rectangle, circle, poly-line or freehand to draw a shape in the scene.
Once you’ve drawn one or more shapes use the toggle tool to make the shape opaque or transparent for players. In the example video below the player’s view is shown on the right: