Lots of interesting stuff happens really fast. Think about a chemical reaction, for instance. The rate of reactions might be slow, but each individual reaction proceeds quickly. This is because a chemical reaction is, essentially, the shuffling of electrons between different atoms, and electrons are fleet of foot.
Generally, if you want to watch something this fast happen, you use what is called pump-probe spectroscopy, in which one short pulse of light initiates an action while another measures the result. A critical requirement for pump-probe spectroscopy is control over the pulses, something that is difficult to achieve in the X-ray regime. This is why a new paper from Physical Review Letters is a promising development.
In pump-probe spectroscopy, the pump is a strong laser pulse that sets a reaction (or action of some kind) in motion. After some delay, a gentler probe pulse measures the state of the thing you just kicked. Repeat this for varying delays between pump and probe and you build up a picture of the trajectory a reaction might take.