A Relations path consists of a number of steps that show the progress of a case.
The steps in a Relations path are called milestones, and they show the progress towards the goal of your case, for example a sales contract. Milestones do not have to be registered in a specific order - the milestone order in the relations path is usually just the most common way of progressing.
Depending on your plan, several relations paths can be created, for example one for Sales, one for Customer Service etc. Each of those relations paths consists of a certain number of milestones. Relations paths are defined globally (i.e. for all teams of a company) by admins. If given the required rights, group and team admins can create their own relations paths.
Whenever you create a case, you choose the best fitting Relations path (if you have more than one) for this specific case. If you only have one relations path, that one will be selected automatically.
To find out what relations path has been used for an already existing case, open the case and look at the case details on top right of the case view.