Answer: Channelrhodopsin is a light sensitive transmembrane protein. It can be used to activate a specific set of neurons.
From https://www.cell.com/neuron/fulltext/S0896-6273(06)00224-8
Channelrhodopsin is an ion channel protein that is light sensitive. The light sensitive character comes from the rhodopsin, which is the light sensitive G-protein coupled protein. It contains an opsin, much like the opsins we have in our visual system to detect light. The name itself is simply a combination of “channel” and “rhodopsin”.
Channelrhodopsin proteins physically change conformation when a photon of light at a specific wavelength hits the opsin. The change in conformation allows the ion channel to open. In classical channelrhodopsin, the ion channel is a non-selective cation channel, meaning that potassium and sodium will move into and out of the cell. The driving force for sodium is very high, so the entry of sodium causes depolarization, which excites the cell.
Currently, channelrhodopsins are used widely in experimental research. They can be used at the whole animal behavior level. For example, putting channelrhodopsin into one hemisphere of the motor cortex causes the animal to activate the limbs on the opposite half of the body. Also, they can be used to study specific circuits at the cellular level. For example, putting channelrhodopsin in a specific region of the brain allows the scientist to examine where that particular region of brain sends neuronal projections into.
Channelrhodopsin was popularized by the laboratory of Karl Deisseroth. The lab was the first to develop a use for the protein in a mammalian system.
The field of potential applications of channelrhodopsins is called optogenetics.