The usability shows how easy it is to fulfil a goal with a system.
A product, software and service all have a usability. The usability represents how easy it is to fulfil a goal with the solution that is used. Multiple components play a role for the usability.
The usability is subjective
A system does not have the same usability for everyone. People who have experience with similar solutions will feel more comfortable using a new solution as they already know the principles from previous usages.
The usability depends on multiple metrics
The good thing about usability is that it can be increased the more the users are involved in the creation process of the new solution. In the analysis phase at the beginning it is possible to get insights into what the users expect and it is also a chance to get insights into the experiences they have.
The usability of a user interface can be increased by building on patterns that users already know, applying proven principles like Miller's Law or Fitts' Law or by simply following the UX workflow and doing usability tests which will instantly result in direct feedback from the users on the solution's e.g. performance.