What is Sinking and sourcing concept?

Sourcing means the Digital Input/Output is providing voltage to the load connected in the closed loop circuit.

Sinking means the Digital Input/Output is receiving voltage from an external Power Source connected in the closed loop circuit.

When digital input/output's common connect to power supply's negative terminal it is called sink, and when connect positive terminal then say source

sinking means the digital input/output provides grounded connection to the load.

sourcing means the digital input/output provides voltage source to the load.