Why trim? It gives your candle a brighter, cleaner burn. Leaving your wick untrimmed and laden with leftover carbon leads to a duller light that's more likely to produce smoke. That smoke can also stain your jar, which honestly screams, "I don't know how fire works!!!"
A good rule of thumb is to trim your wick after every four hours of burn time. When it comes to trimming, always extinguish the flame (obvi), let the candle cool to room temperature, and then trim the wick to 1/8 of an inch. You can trim it with a lil' scissor or spare nail clipper, but there's also this excellent wick trimmer on Amazon for less than $10 that'll make things even easier.