Light sails, aka Solar sails is method to propel a spacecraft using light. The light from the Sun exerts a radiation pressure on these sails that accelerates the spacecraft. However, the magnitude of force is quite low. A sail of dimensions 800m x 800m will experience a force of about 5 newtons at Earth's distance from the Sun.
What exactly happens is that only a fraction of the photons that hit a light sail is reflected. Some of it are absorbed. And this causes a transfer in momentum from the photon to the light sail. For the momentum to be of any considerable value, there has to be a lot of photons and the light sails have to be as light as it can be.
Omuamua is the first interstellar object detected as a passing object through our solar system. Interstellar, meaning that it originated outside our solar system. These objects are hard to detect before-hand that we can be ready to send a spacecraft behind it. In addition to that, the speed of these objects are quite high to keep up with conventional propulsion methods. Light-sails could be a good candidate for probes that can be used to patrol the boundaries of our system for such objects in the future.