Reddit reviews Moen 118305 Replacement Kitchen Faucet Installation Tool

It is to tighten nuts. It goes around the bottom of a faucet underneath the sink. There is a long brass threaded rod that you need to tighten a nut down on to hold the faucet in place. That works better than a wrench or pliers due to the cramped quarters. Source: I just replaced my sink last month. Edit: found one for an example.
https://www.amazon.com/dp/B002PB1JJS/ref=cm_sw_r_cp_apa_i_9sXCDbK5RF95Y

Faucets usually come with a plastic wrench for nuts like that, it's kind of a deep well plastic socket.

Example

http://www.amazon.com/Moen-118305-Installation-Tool/dp/B002PB1JJS/ref=sr_1_6?ie=UTF8&qid=1463958494&sr=8-6&keywords=plastic+faucet++nut+wrench

Plumbers save a few different types to carry in their tool boxes, the one linked might fix, but if you can find one for that brand faucet it would be better.

Moen 118305 installation tool
https://www.amazon.com/gp/aw/reviews/B002PB1JJS/ref=cm_cr_dp_mb_see_rcnt?ie=UTF8&s=sd

Former Moen engineer here. You need one of these.

Moen 118305 Installation Tool https://www.amazon.com/dp/B002PB1JJS/ref=cm_sw_r_cp_api_i_XR.SCb5FDHZ4E