When most people think of SETI they think of the radio telescopes that search for signals from extra terrestrials, but since we have no facts about how these aliens might choose to communicate and so we search in every way that we can think of. One of the ways that SETI has found to search is optical SETI which searches for signals the same as radio SETI but searches for light signals.
These signals would almost certainly be lasers of some type and unlike the typical signals that SETI searches for since lasers are focused they would have to be aimed at earth. This means that while radio SETI could potentially find signals from a species that was simply broadcasting TV over the airways similar to how we do, but a laser would have to be aimed at us.
Because of this the odds of SETI discovering messages through optical telescopes is far less, but the meaning behind that message would be far more profound. It would almost certainly mean that the species was more advanced than ours, able to recognize that there was an intelligent species here as build a laser that was powerful enough to contact us.
While the disadvantages of optical SETI keep it in the position of a far less important part of the SETI experiment it does have some significant advantages over radio SETI. The most important of those is that unlike radio SETI there is no interference from earth based signals meaning that if a signal was found it would be far more likely that it would be a mistake. This alone will makes optical SETI worth testing.
The most recent experiments use three photomultipliers that search for photons from a distant that arrives within less than a billionth of a second apart. Using three allows for a reduction in the number of false positives but it is still possible for bright stars to trigger the sensors.
Optical SETI has been tried before in the past but always at a much smaller scale. In the most modern attempt thousands of systems will be searched. Though this may sound like a large undertaking the truth is that thousands of system is tiny percentage of the stars in our galaxy and the job of SETI whether with the use of radio or optics is likely to continue through our lifetimes and longer.
No matter what SETI is searching for it is the search that matters more in many ways than the discovery. Humans are a species that has always looked out over the horizons searching for others like ourselves and like the campfires that once signaled our ancestors of others like them win distant hills perhaps lasers will signal to us that there are others like us on other worlds.