Zero sprint can be defined as the pre step to the first sprint. And jobs such as setting up the environment to develop, prepare backlog, etc. and that needs to be performed before the start of the initial sprint and can treat it as Sprint zero.
That said, as the demand for Agile grows, so has mystification around the term Sprint Zero. Most often, people think of Sprint Zero as applying the framework of a Scrum Sprint to the pre-planning process for a project whereby the pre-planning stage becomes a project in and of itself during the sprint. This is sometimes referred to as “the project before the project”.