You create new route feature classes in ArcCatalog. When creating a route feature class, you must define the feature class type as line and indicate that it will include m (measure)-values.
You also need to add a route identifier field in the feature class. This field uniquely identifies each route.
When you are performing linear referencing operations in ArcMap, you will select a field in your route feature class layer that will supply this unique identifier. The value in each field is used to locate events on the line features. First the route ID is matched to a feature, and next, the event table's measures are used to locate the event along the route feature.
You can read more about how the route ID is used at Using a route identifier field in ArcMap.