OK, this is slightly nasty and combines a couple of more difficult points of grammar.
The first is the form l'on, which is essentially a formal version of on (meaning "one", "people in general"). Essentially, writers tend to use l'on in formal use when it "sounds better". (I can go into more details if it's interesting, but essentially that's the idea.)
Then, mette is a special form of the verb mettre. This special form is called the subjunctive, and is essentially used in cases where you're not actually stating that something happens/happened, but more like taking a "snapshot" of a situation. For more details, have a look at the site's section on the French subjunctive.