In William Shakespeare's play As You Like It, the character Jaques delivers a monologue about the different periods in a man's life, which he refers to as the "seven ages" (that are like the acts of a play).

The fourth age is that of an adult man, who is a soldier or behaves like one. He is prone to jealousy and will be "sudden and quick" when he is arguing. The effectiveness of the personification of the cannon's mouth lies in the phrase just preceding it. This rash man will seek "the bubble reputation / Even in the cannon's mouth," meaning that he aims to enhance his own reputation in a quick and showy but insubstantial way; it will blow up fast and burst like a bubble.

By saying that he seeks this inside the cannon, Jaques emphasizes both the man's identity as a soldier (who might be operating a cannon) and the contrast between two ephemeral things—the bubble and the cannon's explosion. There is also a contrast between the fragile bursting bubble and the large, solid weapon.

I am not sure what there is to explain here.

The "mouth" of a cannon is simply the opening in the front of the cannon where the cannon ball would (in those days) have been put in and where it would come out when fired.

The personification here is probably meant to convey the danger that the man is willing to face in trying to be a hero.  When we say that the cannon has a mouth, it sounds like it is going to eat the man. So it sounds very vivid -- he is going to attempt to be a hero even in the mouth of the cannon (by charging right up to the cannons in war).