Utility is a mathematical description of how favorable an option is. Utilities are unit-less, and often expressed as values between 0 (no utility) and 1 (highest utility). Sometimes, utilities are expressed as negative numbers to represent the burden of an action. For example, the utility for taking the bus can be more negative than the utility for taking a car in a mode choice model to express the added burden of waiting for the bus, maybe transferring, and maybe longer travel times on the bus.
Further reading available on Wikipedia.