Evans is Windsor's local parson (aka clergyman) and the town schoolmaster, and he loves being involved in the day-to-day lives of Windsor's citizens. He makes it his mission in life to help Slender win the heart of Anne Page (1.1) and even gives little William Page a Latin grammar lesson on his day off (4.1).
Even though Evans is an active member of our little community, his status as a foreigner makes him somewhat of an outsider in Windsor. Take his seriously thick Welsh accent. It seems like every time we turn around, someone's making fun of his speech (when they're not making fun of the Frenchman, Doctor Caius, of course). That's why Falstaff complains that Evans "makes fritters of English" (5.5.135).
In the play, Doctor Caius challenges Evans to a duel for trying to help Master Slender woo Anne Page. Before Evans and Caius can throw down, the Host of the Garter Inn prevents the fight and declares "let them keep / their limbs whole and hack our English" (3.1.66-67). Translation: it's more fun to listen to the two foreigners (Caius and Evans) butcher the English language than it is to watch them "hack" into each with their swords.
(In Merry Wives, trash talk and verbal sparring take the place of actual physical violence. More on this in "Themes: Language and Communication.")
Evans is a pretty smart guy so, eventually, he realizes that the Host is having some fun at his expense. That's why he and Doctor Caius hatch a plot to get revenge on the Host by tricking him out of a bunch of horses.