—Information accurate as of: build 904.83
The Holosight is a ranged weapon attachment which adds a holographic sight to the weapon it is mounted on. It helps make it easier to aim the weapon at a target.
To mount a weapon mod, simply open the inventory, and drag the weapon mod onto a weapon mod slot or straight onto the weapon itself in the item panel. Weapon mods can be dismounted by dragging the mounted mod off of the item and back into the inventory/storage.
To scope in with most guns, regardless of the mounted weapon mod(s), hold the right mouse button.
The Holosight can be crafted with:
It takes 120 seconds to craft the Holosight.
There is some info on the Holosight in Devblog 77 of the official Rust website.