The term "mlp-applejack" is a porn video tag related to a specific category within adult content. Here's what it means:
MLP stands for "My Little Pony", which is an animated television series and franchise created by Lauren Faust, based on the Hasbro toy line of the same name. Applejack is one of the main characters in the show, known as a kind and hardworking pony with a strong sense of morality and justice. She is often depicted wearing overalls and wielding a shovel or a pitchfork.
The tag "mlp-applejack" is used to describe adult content that features this character from the My Little Pony series in explicit and sexual scenarios, often in an anthropomorphic form. This category is for mature audiences who are specifically interested in My Little Pony-themed adult content involving Applejack.