in computers and microcontrollers data is transferred in two ways, serial and parallel. so serial communication protocols mean data/bits are transferred in series. meaning first bit is transmitted then second bit transmitted and one by one until bit 8 is transmitted to send 1 byte of data. examples of popular serial protocols are I2C, RS232, USB, SPI, …
on the other hand parallel communication protocols send 1 or more chunk of data (in form of byte/bytes) in one time. for example, if i want to transmit one byte i will need 8 wires between transmitter and receiver so we can transmit each bit in wire. examples of parallel protocols are ATA, SCSI, PCI, …
