What is the lore of Monster Hunter: World?

Solution 1:

This answer doesn't contain spoilers, this is all mentioned in the first few cutscenes of the game.

The New World is an uncharted continent, not inhabitated by humans, which is in the process of being explored by a division of the Hunters' Guild called the Commission. The Guild has sent five fleets of hunters, technicians, and scholars to the New World over the years, in hopes of solving the mystery of the Elder Crossing. You are part of the fifth fleet.

The Elder Crossing is the name given to a phenomenon in which elder dragons migrate to the New World in waves. This used to happen approximately every hundred years, but it has changed a few decades before the start of the game, with elder dragons migrating as frequently as every ten years. The entire purpose of the Commission is to find out why elder dragons come to the New World, the most recent one being Zorah Magdaros.

The scoutflies are adept at tracking a monster's scent, and have been trained to guide hunters to the monsters they're following.

Solution 2:

The Monster Hunter franchise is not one known for lore. It is known for hit monster until it stops moving and then wear its skin as a hat. The lore is only in place to get you doing that.

That said, World is the 5th major entry in the series, and has a bigger focus on story/lore than its predecessors.

You are a new Hunter of the fifth fleet on an expedition to the New World, a newly discovered continent. The Research Commission has summoned hunters to research and catalogue all they can find on this new region, and study why Elder Dragons seem to flock here every 10 years or so, known as the "Elder Crossing".

As you play through the story quests of the game, you learn more about this world, the elder crossing, and other secrets it has tucked away.

Bonus: The scoutflies are special creatures/tools provided by the Research Commission to assist the Hunter with tracking and research.