You'd be pretty soggy by morning if you accidentally set your tent up in a bog. A bog is a swampy kind of ground made up mostly of decomposing plants and mosses.
Nothing can be built on a bog because the ground is so spongy and damp, and no crops can be grown there. The dried soil from a bog can, however, be dried and burned for fuel. A figurative bog is anything that you tend to get stuck in, like a bog of health problems. Things can also "bog you down", or slow you down literally or figuratively, like when your homework bogs you down so you can't enjoy the weekend.