CAN bus is used for data transfer between several electronic devices in a vehicle. Generally, data is sent by one device(transmitter) in the form of a data frame which is received by another device(receiver) for which the data is meant.
Also, the receiver can itself request for data from the transmitter. For this, the receiver sends a frame with no data, also known as remote frame. This remote frame has the same structure as the data frame except for the fact that there is no data present and the RTR bit is 1.