The Host is the major mover and shaker of the frame story of The Canterbury Tales, since it's he who proposes the tale-telling game and directs it on the way to Canterbury. We get the impression that the Host is a jolly fellow – large, with piercing eyes, fit "to been a marchal in a halle" (General Prologue 754), which is in fact what he becomes as the self-styled director of the pilgrims' merriment.
The Host just wants to have a good time, which is why he gets so upset with pilgrims like the Monk or the Physician, who tell stories about tragedy and death, and so delighted with those like the Nun's Priest's, whose stories are comedies. The Host takes all his tales extremely seriously, becoming very emotionally involved in them. So convinced is he of their power that, after hearing the Physician's mournful tale, he declares the only thing that will save his heart from pain is "a draughte of moyste and corny ale / or but I heere anon a myrie tale" (Pardoner's Introduction 29 – 30).
He's also quick to draw lessons from the tales he hears, declaring that the Physician's tale proves that Nature's and Fortune's gifts are death to every creature, or that the Merchant's confirms lots of negative stereotypes about women. The validity of the Host's conclusions is questionable, though, which makes us question his intellect, though not his enthusiasm.
The Host often tries to play the role of peace-keeper among the pilgrims. He's the one who tells the Friar to stop ribbing the Summoner, for "in company we wol have no debaat" (Friar's Prologue 24), or warns the Manciple to stop insulting the Cook lest his insults rebound upon him. Since his goal is to keep all the pilgrims having a good time, it makes sense the Host tries to be the dispeller of conflict among them.
But sometimes, he creates as many problems as he solves. Take that incident with the drunken Cook, for example: the Host is the one who calls him out for drunkenness in the first place! Or look at the moment when the Pardoner tries to foist off his fake relics on the pilgrims: there, the Host responds with outrage and anger, necessitating the Knight's mediation to avoid conflict. At these times it's like the Host's strong emotions and desires get the better of him – he simply can't contain his hilarity at the Cook's drunken state, or his righteous anger at the Pardoner – and prevent him from fulfilling his goal of promoting merriment.
With the Host, The Canterbury Tales have a built-in audience. What do we mean by that? Well, the Host is the pilgrim we most often see responding to the tales, which makes him kind of a mirror image of us, the reader, who are responding to them, too. It's doubtful that the Host is an ideal responder, since his interpretations of the tales are often so off-base. But those very wrong interpretations make us, the readers, think more about what the tales really do mean. And if they do that, then the Host has served his most important purpose.