Translation:I find it difficult to believe you.
Personally I would never use the phrase in the example, I would say either Jeg synes det er vanskelig å tro deg or Jeg har vanskelig for å tro deg or Jeg har problemer med å tro deg.
Å finne means to find. Å finnes means to exist. S-verbs are peculiar... they get the s from being words of a certain quality/activity/duration different from the stem. One of the easy examples: å slå to hit, å slåss to fight.
S-verbs are verbs that look quite similar to and derives from a regular verb (but with certain altered meanings and uses), but if the infinitive and present form doesn't end in -s it's not an s-verb. Å finne - finner. Å finnes - finnes.
Å finnes = to exist. Jeg finnes = I exist/am existing. Jeg fantes = I existed (S-verbs have other forms, it's not just the s)
Å finne = to find. Jeg finner = I find/am finding. Jeg fant = I found
En finne, to finner = a fin, two fins/a Finnish person, two Finnish persons (both a fin like on a fish and someone from Finland)
Finner finner finner? = Does Fins find fins?