The Thai chili, also known as bird’s eye chili, is a common chili pepper found in Thai and other Southeast Asian cuisine. They’re small, but pack quite a bit of punch! It measures 100,000 to 250,000 Scoville units (heat scale) which is on the lower end of the habanero range – pretty hot!
They vary in color from green, to yellow/orange, to red and are fantastic fresh or dried.
Everyone has their own personal heat level that they find comfortable. I have a relatively high threshold for heat so I like to use quite a few in my recipes, however always defer to your heat preference. As a note, removing the seeds and will significantly reduce the heat without loosing the flavor of the chilies.
Many general grocery stores carry Thai chilies these days and pretty much any decent Asian grocery store should carry them as well. If you cannot find them, you may substitute cayenne pepper or Serrano. Just be aware that they have less heat respectively, so you may need to use more.