What is the meaning of VIA ATA Channel in the "Device Manager" of my computer? Why is there a yellow exclamation point in front of it?

VIA is the name of the company that makes the chipsets on motherboards.

ATA is a hard disk interface.

Channel refers to the hard disk transfer data channel.

The yellow exclamation point means that the "driver" for your device is not installed, the solution: install the motherboard driver.