What is an appender?

An appender is the term used for anything that emits a logging statement to a destination. For example, a logger that writes logging statements to a file is referred to as a file appender, and a logger that writes to a MongoDB database is a MongoDB appender (which natively exists in Timbre). Basically, appender is a fancy pants term for "writer". In the world of Timbre, an appender is, technically, just a map of options, one of which is the appender function responsible for performing the actual logging.