The Kobra sight is an optic in Battlefield 3. It is identical in function to its American counterpart, the Red Dot Sight, but it has 3 intersecting red lines, pointing downwards from the center, to the left, and to the right.
It is unlocked as the first optical attachment for many weapons originating from Russia and many other weapons, sniper rifles where 12x Ballistic is always the first to be unlocked. It is also unlocked for American and most NATO weapons, such as M16A3, SCAR-H and M98B relatively late, at approximately 200 kills.
The Kobra is ideal for close quarters combat due to its quick aiming speed and clear sight picture. Many players prefer it over the RX01 reflex sight due to its frame being much less bulky and hence being less intrusive on the player's peripheral vison, and for its unique reticle not blocking out the center of the player's target, hence making it clear to see exactly where it's aimed. Choosing one over the other is simply a matter of personal preference.