Timbre is a full Clojure library for logging. It's fast, has low overhead, and has many interesting appenders out of the box. Most importantly, setting it up is relatively easy and straightforward compared to other logging libraries.
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.