what is this brown viscous fluid that builds up in my dry herb vape?

That’s oils and waxes from the bud that condense on the sides of the glass and build over time. You can clean it off by using isopropyl alcohol 99-100%. It’s also possible to reclaim it and ‘eat’ it providing potent results. Check yt for guides.

Worth mentioning that using isopropyl and eating it are almost mutually exclusive. Use everclear if you wanna safely ingest. Always research extensively when flammables are involved.
Also lower % iso is usually better, 70-90 is more better than 99

reclaim. best way imo is to throw the glass into a pot of milk and boil it until the reclaims comes off (then wash the glass, obv). when the milk cools add it to coffee etc.

That's a great vaporizer. I use the same one.

I was very lucky to pick up the airizer max while half off on the spring sale. I like it so much I considered buying a second one but that $ is better used right now.

This method sounds interesting! By boil do you mean like a low simmer? I am worried about damaging the glass via thermal shock or something

Rise the temp of the milk to boiling temp, the glass won't care.
- Thanks!
Some people gently simmer the stems in whole milk - the fat in the milk binds to the vapor reclaim in the stems. After "cleaning" the stems you remove from the milk (the will be fine to cool down naturally on their own - just don't throw them into cold water instantly lol). Take that milk, throw in some chocolate and make yourself a sedative hot-chocolate drink!
- Thank you for the clarification! I will try this!

What's a dry herb vape?

A device that heats cannabis flower to a temperature that volatilizes the terpenes and cannabinoids, but does not allow for combustion of the material
I looked to find a decent 101 on dry herb vapes but alas the enshittification of the internet continued and I couldn't find any sites that aren't also trying to sell something.

So in addition to wht others have explained, here's a bit of extra info from the best two pages I could find:

https://www.planetofthevapes.com/blogs/blog/how-to-vape-dry-herbs-for-beginners

https://thevape.guide/how-do-dry-herb-vaporizers-work/

Dry herb vaping also advantage of working with plain flower so you know what you're vaping as opposed to refined vaping products like juices, waxes, oils, etc. which can have additives that aren't so good for you.
Instead of lighting your herb on fire to smoke it (combustion) vaporization uses an electric oven to bake your flower to a specific temp at which the cannabanoids release as smoke while the flower slowly bakes (vaporization). The resulting smoke is a lot cleaner on your lungs and you can get a lot more out of it. .2 g is 10 minutes of smoke in a vaporizer while 10 seconds of smoke in a bowl

The resulting vaporized herb is also a useful byproduct. Its effectively poorly decarbolized flower that you can use to make cannabutter with.

Yup, that's pretty much what it is. Reclame.