I'd learn the basics of the QGD & QGA before entering the bonkers world of the Dutch Defence if I were you.
But, to put it in simple terms Nf6 guards the central e4 & d5 squares.
I'm no expert & only score about 50% with the Dutch, but here's a mainline:
white to play
The Nf6 allows black to pretty much own e4, along with being ideally placed to go Ng4 threatening f2 or Nh5-Nf4.
The knight move also uncovers fianchettoed Bg7.
Black tends to get chances to expand on the KS at the expense of at least partly conceding the QS.