It's an open letter from the pope. An encyclical will often clarify Church doctrine on a topic, or set of topics. See e.g. Humanae Vitae[1] for an example of one that has resulted in, essentially, a lay rebellion — at the time it was issued many clergy, including Bishops opposed it, but the Church itself has gradually become far more conservative (perhaps as a reaction to Vatican II) in the intervening years.

1: https://en.wikipedia.org/wiki/Humanae_vitae

Just a letter from the pope, often about how Catholic teaching relates or applies to some modern issue. They present nothing new in terms of Catholic teaching itself, but, through the pope's authority, serve as important guidance for the faithful.