Omni-roast means that the roast profile has been tailored for the coffee to taste good both brewed as filter coffee and espresso. This is often done by reducing heat in the roasting process to give the beans a longer development time.
The benefits for the roaster is that you can match a broader taste profile with one roast and also reduce the risk of disappointed customers who may have used another brew method than intended. For example a fruity light roast filter coffee can taste really unbalanced and acidic/sour when brewed as an espresso. Omni-roast is also great for less experienced baristas or for roasters who encourage their consumers to experiment with different brew methods.