What's this device (circled in blue) that's attached to my furnace? It recently got replaced and I forgot what the HVAC technician called it, or what its function is.

I do rember that they said to put some vinegar in the U-bend (circled in red) once or twice a year. I forgot to ask why this is necessary, but I'd guess that it tends to collect moisture, and the vinegar will prevent mold?