The vision tool can be accessed in the toolbar on the bottom right of the Game Board in both modes. It helps to track vision of multiple tokens. Its behaviour is different for players and DMs.
If clicked, the Vision tool will open a list of all objects that are configured to be a token and that the player has vision access to. By default, they will be shown active in the list, denoted by having a coloured background. All modes and tools will only show the vision of those tokens that have an active entry within the Vision tool.
On click, an entry will be deactivated, showing now a grey background-colour. Simultaneously, the vision of this token will no longer be shown on the Game Board.
By default, DMs always see everything. Using the vision tool alone will not change that. They can, however, fake player to achieve a similar effect to that described above. In this case, they use the vision tool to define which player's vision they want to see.