NI DIO serial or Parallel

Hi, 

What is mean by Serial Digital Input output module and Parallel Digital Input output module.

Serial Digital Input module not support counters, eg:NI 9375

Parallel Digital Input output module support Counter operation 

The link is below saying serial and Parallel type for DIO module

https://www.ni.com/en-in/support/documentation/supplemental/18/cdaq-module-support-for-accessing-on-board-counters.html

What is mean by Serial /Parallel type DIO module