When using AYNC communications over an RF link it is often difficult to ensure that the receiver is properly syncronized at both the byte level.
Synchronization at the bit level means that the receiver knows the time when each individual bit in the received data starts. This typically happens automatically as the UART looks for start bits.
Syncronization at the byte level means that the receiver has properly determined where the start and stop bits are, and thus how the bytes are framed within the message. This is more difficult because the transmission often begins with a preamble, or noise, that prevents the UART from seeing the first start bit correctly.