DC Optimizers are essentially very small inverters that sit underneath the solar panel, hidden from sight. Each solar panel has it's own optimizer.
The optimizers convert the AC power produced by the panels to DC power so it can be used by all of your household appliances. It is much more efficient than a single inverter as it allows for the monitoring of each individual panel, which means you can recognize and respond to any issues that may arise very quickly.
Optimizers also help to get the system ready for a battery as the DC wiring goes all the way to the central inverter.
Optimizers allow for larger strings of panels to be created, which means the system overall is cheaper as a larger string of panels is much cheaper than a smaller one.