VOCs -- i.e., volatile organic compounds -- are chemicals with molecules that can evaporate very quickly when exposed to ordinary room temperature.
Some VOCs are a problem because they can damage an area’s indoor air quality (IAQ) by causing discomfort through foul odors or by straining people’s sinuses, eyes, skin, etc.
The Environmental Protection Agency (EPA) states that VOC concentration can be up to 10 times stronger indoors than outdoors. VOCs that are harmful to IAQ include acetone, acetic acid, butanol, ethanol, carbon disulfide, formaldehyde, and methylene chloride -- see here to understand how and why.