The story is set in the USA, in the period of about 1830/40. This was before the American Civil War (1861-65) which was fought (mainly) over the issue of slavery. So, at the time of the book, slavery was a way of life. This mostly concerned the Southern States, but there was an uneveness throughout those states in the harshness with which the laws and customs of slavery were applied. Generally though, a slave was regarded as the property of his or her owner, that is the person who had purchased them. A slave who ran away was punished very severely indeed, lashing was the 'easiest' of those punishments. Death was a possibility, but not too frequently used since it robbed the owner of a slave, but life was made very cruel indeed for a recovered slave.

There seems to be an anomaly in the story in that, when Huck runs away with Jim (a slave) — which was a very serious crime — they head south instead of north which would have offered freedom. The reason for this was their mode of transport — a raft. With no power they could only go with the flow of the river, which was south — to the sea.

There are 7 towns called Cairo in the USA, but the one mentioned in the story lies at the meeting of two rivers. One is the Upper Mississippi (which Huck and Jim were coming down), the other is the Ohio river. After they merge at Cairo they become the Lower Mississippi.

Hence the importance of Cairo. The idea would have been to get off the raft at Cairo and (somehow) get on a steam-boat heading north up the Ohio river — to freedom. So passing Cairo in the night, as they did, would have seemed like a disaster.