The video shown on the right is a video of an addon prop from the game Penumbra: Black Plague.
Addons for Garry's mod are mods in the game. Addons can be either maps, or props.
Before the steam workshop, you would install prop addons by simply dragging them into the addons folder. You could install maps by dragging them into the maps folder.
NOTE: In the most recent version of Garry's mod, (June 2013 and newer) installing addons as explained above does not work! You must install them from the Steam Workshop, which is much more simple. The player just clicks the + button and the addon is being installed. This is all done ingame.