Agincourt was a decisive military victory by a beleaguered and numerically inferior English force against a more powerful French army. It was a crushing defeat for the French, who may have lost up to 10,000 dead (in comparison to about 100 English killed).

It was, perhaps more than anything, a huge propaganda victory for Henry V, who came home to Englishmen literally singing his praises. Understanding why requires one to recognize that Henry's campaign in France had, up until this point, been something of a bust. The siege of Harfleur took much longer than expected, and during it the English were hit hard by disease. Though the English army took the city, they spent the next few weeks being ineffectually being pushed around by the French while trying to cross the Somme.

When the battle began, the English were exhausted, far from home, sick, and low on supplies (like food). They by rights should have been slaughtered en masse, but instead turned around one of the most lopsided military victories in history. It was a contributing factor to the civil war between the Burgundian and Armagnac forces, which in turn helped Henry V to victory.

A good popular source for this is Julia Barker's Agincourt: Henry V and the Battle That Made England, 2007.