Rider Passes are the Kamen Rider: Electric King system's ticket system for transformation & transportation through time via Electro-Liner.
When it is scanned on the Electro Belt, or any similar belt system belonging to time trains, any Rider or Imagin is allowed access to his personal Rider form, armoring him up & providing him access to the ElectroGasher.
The pass can store tickets scanned from memories of Imagin victims. The pass, with the ticket is inserted on the Electro-Bird console as it is docked with Electro-Liner. It allows the Electro-Liner to travel to that time period.
Initially there were only two passes on the Electro-Liner, the Rider Pass, which belonged to Megan, which she lost & eventually made Dwayne keep, & a gold Master Pass, which belonged to the Owner (which Kiba eventually stole). The first incident where there were more than a couple of passes was when Garrett Meggyesfalvi brought in several Dwayne Winfields from different points in time to battle Kamen Rider Fang King. Later on, it was revealed that Negataros had stolen a spare Rider Pass, & reverse-engineered it to duplicate the Electrc King system in its entirety. To battle Kamen Rider Steam Ghost, Megan provided each of the Taros a pass, in addition to Dwayne & Henry's passes.
Kamen Riders Minos, Fang King, & Steam Ghost also have their own Rider passes to activate their own battle systems, similar to (& likely based on) Electric King's system.