Sink Current

Sink current is the ability of the digital output/input port to receive current. When you have a simple circuit where a digital input connects to a digital output, you need three components; a voltage source, a ground, and a load. A sourcing digital I/O provides the voltage, a sinking I/O the ground, and the digital input provides the load.

This means that the input and output works in pairs:
Digital source output – Digital sink input
Digital sink output – Digital source input

Sink Current is the maximum current an amplified circuit can accept (“sink”) on its output pin and still remain within the specified performance limits.