I noticed a small hole on the lower left side of my iMac. Can anyone tell me what it is for?

If it is a new 24" M1 iMac, then that is the Headphone port.

see > Take a tour: iMac - Apple Support

If it is not a new 24" M1 iMac, then please identify the exact iMac year model.

see > Identify your iMac model - Apple Support

It’s not the headphone jack, as it’s (at least, per the footer) running macOS 10.13.

Which iMac model? Identify your iMac model - Apple Support

On most (all?) iMac Intel models with the chin, the speakers are downward-facing on lower left and lower right.

You're welcome.