The above lines come from Jaques's famous "Seven Ages of Man" monologue, addressed to Duke Senior in act 2, scene 7 of As You Like It. In this, he refers to different stages in a man's life.

When he comes to discuss the fourth age, which is that of the fully-grown adult male, he likens man to a soldier, "Jealous in honor" and with a quick temper that often leads him to get into quarrels. "Seeking the bubble reputation," a reputation that is as fleeting as a soap bubble, he throws himself into battle, risking his life in front of "the cannon's mouth."

Here, we see a prime example of personification, the attribution of human features and qualities to things that aren't human. A cannon isn't a human and doesn't really have a mouth, but the big black hole of a cannon certainly resembles a mouth, albeit a toothless one, so the expression is by no means inappropriate.

What Jaques is driving at here is that man in his fourth age is so determined to achieve a good reputation at this time of life that he's prepared to take dangerous, foolish risks. And for what? Something not destined to last, something that is as ephemeral as a bubble, here today and gone tomorrow.