Pollution

Pollution is a fairly large aspect of the game, especially for newer players. The reason I say this is because Pollution affects the evolution of the enemies and how strong/often their attacks will be. Nearly all buildings in Factorio produce some amount of pollution while they are actively working. You can tell how much pollution something generates by mousing over the building once it is placed down.

Things like Boilers, Burner Miners, Electric Miners, and Furnaces produce a very large amount of pollution. Some things don't produce pollution themselves, such as Steam Engines, or Electric Inserters. You can see a visual indicator of how much pollution is in the air at the time by havieng the Advanced Info on (ALT). It will show a brownish redish cloud of pollution on your minimap and the main map if you have the Advanced Info on. You can see where it has spread and is continuing to spread. The darker red parts are where the heaviest pollution is, and the lighter areas have less pollution in them.

The numbers that indicate how much pollution something produces actually mean something, and how pollution spreads is actually a pretty intricate process. It is also a bit complicated as to how the pollution actually affects the enemy evolution and attacks. It would take a while to explain it here. So below I will give a link to a video that goes over it in great detail, as well as the Factorio wiki page on Pollution.

This video goes over pollution in great detail. There is also a simple explanation at the beginning of it if you aren't interested in the in-depth stuff.
This Factorio wiki page is just the on Pollution. It explains it pretty well, and is usually up to date.

Hopefully between all three of those pages, you should have a good understanding of how the Pollution works in Factorio.