This is an Eccentric spacer that is used to tension V-Wheels against the MakerSlide rail. The spacer can be mounted in a 7.14 mm (0.281” or 9/32” dia.) hole so that the inner 5 mm hole can be rotated, adjusting the pressure of the V-Wheel against the MakerSlide. It is 6.35 mm (1/4") tall, and can be used in between the wheel and carriage plate, or on the outside of the carriage plate. If it is used in between the wheel and mounting plate, the other fixed wheels will need to have 1/4" spacers, to account for its height.
Other information (from the V-Wheel description)
The wheels need to be mounted on a carriage (steel/aluminium/plastic plate). The standard configuration would be to have four wheels per carriage; two wheels would be fixed in position with a M5 bolt, the other to would be adjustable via an eccentric spacer. The distance from centre to centre for two wheels to run on a piece of MakerSlide is 64.4mm, one of the mounting holes needs to be 5mm (for the fixed wheel) and the other needs to be 7.14mm (9/32" or 0.281") for the eccentric spacer. When mounting the wheels the eccentric spacer can be positioned on the inside or outside of the carriage plate, if it is positioned on the inside then 1/4" spacers will be needed for the opposing fixed wheels. Once assemble is complete, adjust the eccentric spacer so that the wheels are the widest distance apart, then slide the carriage onto the MakerSlide. (Having them as far apart as possible will reduce the bump they have on the edge of the MakerSlide; if they do have a bump with the sharp edge it may damage the wheel shell.) A good tip would be; have the weight bearing wheels as the non-eccentric fixed wheels. Using a 8mm spanner, adjust the position of the eccentric spacer until the desired tension is reached. The tension does not want to be too much so the carriage can barely move, but not too little so it is not unsecure. A good rule of thumb is to have the carriage so that when dangled vertically it doesn't drop off the MakerSlide but is nearly about to. You also have to take into account the strength of your carriage material, if you are using something flexible like a piece of laser cut acrylic, the carriage could bend when the eccentrics are over tightened.