Thais tend to eat often and so you'll see a great deal of edibles being sold on the street in Chiang Mai. Food stalls come in all shapes and sizes, ranging from basic push carts to elaborate rigs that operate from the back of a large vehicle.
Some only do takeaways whereas others provide tables and chairs for a real roadside experience.
As to whether these places are hygienic or not, well .. it depends. Some are and some aren't. To be fair, some may be cleaner than a four-star hotel restaurant. A good guide is to look at how clean a vendor's work surface is before ordering anything. How many flies are on that slab of pork?
If everything is covered in grease or grime then go to the next one along. They usually hang out in groups.
Another thing to watch for is where do they do their dishes? In a pan on the ground with dirty water? Or on a table with clean running water?
The fare is not usually outstanding but suffices to satisfy most rumbling tummies. But then what to expect for 20/30 baht?