Chiggers are an immature stage of certain types of mite species. When chiggers attach to the skin, they release a digestive enzyme into the skin that liquifies skin cells. This enzyme causes rashes that are extremely itchy and cause a great deal of misery. Sometimes the rashes will show up after a few hours of being fed upon by the chiggers.
Chiggers tend to attach to the skin where it is thin and tender. They will also commonly attach to the skin where clothing is compressed against the skin. For that reason, many of the marks and rashes show up around the ankle where socks are tight and also around the waistband of pants or shorts. Contrary to popular myths, chiggers do not burrow into the skin, carry diseases, or feed on blood.