In the Thai language, small hot chili peppers; literally "mouse shit peppers". They are often called bird chilis or birds eye chilis in English, though I usually just call them small hot Thai chilis.
Prik kee noo are one of the many kinds of peppers used in Thai cooking, and as sneff has pointed out elsewhere, these and other chilis were introduced into Thailand (then Siam) by the Portuguese in the 17th century. Previously, spice in Thai food came primarily from peppercorns, called in Thai prik thai. As with many things, the Thai showed their adaptability by the way they were able to incorporate foreign objects/ideas/customs into their own lifeways in a way that makes these things quintessentially Thai. Prik kee noo are a case in point, and they are now ubiquitous and totally at home in Thai food.