THHN is a rating which describes the insulation around electrical wire. Included in the rating is what material is used, how hot it can safely become before it breaks down and how easy the wire is to install. The National Fire Protection Association (NFPA) publishes safety ratings for electrical products.
THHN is an acronym for Thermoplastic, High Heat resistant, Nylon jacket insulation. This is one of the most commonly used insulation on wire.
Thermoplastic is a PVC material, which makes it a thinner insulation than most other types. Thermoplastic also emits toxic smoke when it gets hot enough to burn, so the installed wire shouldn't be exposed.
It is important to know the temperature at which the insulation will break down. HH means the insulation will withstand a 90 C (194 F) temperature.
Protecting the thermoplastic is a nylon jacket. The nylon also makes installation easier because the wire will slide freely through conduit.
Whether the wire is copper or aluminum, stranded or solid, it is meant to carry electrical power from one place to another. THHN is a description of the protective insulation that covers electrical wire.