This is a feature in the browser version of Agar.io which allows the player to zoom out and zoom in. Zooming out allows players to get a larger view of the map, with many more cells in view. Zooming in allows players to get a more condensed view of the map, with fewer cells in view. To change the level of zoom, use the mouse wheel. The level of zoom is relative to the number of cells of the player and their size.
In the spectator mode it is possible to unlock he camera so that it is no longer tied to a player. In this case the zoom level has a fixed maximum.