Keeping aerobic conditions throughout your bin is crucial to a successful worm bin.
Aerobic means "in the presence of oxygen."
Anaerobic means the opposite - "an absence of oxygen."
A soil, or compost, should have 6 parts per million of oxygen present at all times to be considered aerobic.
The microorganisms that exist in aerobic conditions are generally the ones that are beneficial to plant growth. Most disease causing or pathogenic microorganisms are found among anaerobic organisms.
Anaerobic conditions will lose nutrients from the metabolic processes of anaerobic microbes (the bad smell is the smell of nutrients being gassed off), potentially harbor harmful bacteria, be high in acidity, and can harm or kill your worms.
Aerobic conditions will do the opposite. It will keep nutrients, kill or outcompete harmful bacteria, remain pH neutral, facilitate quick decomposition, and keep your worms happy and healthy!
Without a microscope, there are other easy ways to get an idea if your worm bin has anaerobic conditions or not.
Aerobic conditions will
Be free of pests like flies and gnats
Feel fluffy, spongey, or crumbly like a healthy soil
Happy, healthy worms
Anaerobic conditions will
Potentially have pests
Feel gunky/slimy or waterlogged like clay or mud
Unhappy, unhealthy, or dead worms
Aerobic conditions is something you create by properly managing the other factors such as not overwatering, overfeeding, and using plenty of bedding that, by its physical structure, allows for air to permeate throughout the bin.
So, to be well prepared to keep an aerobic worm bin, read through the entirety of this post and the posts that it links to!