The state of nature is the condition we all would be in if government did not exist. The way it's sometimes presented, it's the condition before the rule of law comes into being. Some have thought that there was a time before any government, any official monopoly on the initiation of the use of violence, came into being.
What we know about the social behavior of indigenous peoples in undeveloped countries shows that that might be wrong in point of historical fact. It is very rare, indeed, that a group of people lacks anything like a government at all, even if the "government" consists only of tribal elders. That's why I say the state of nature is a "useful fiction." More complex governments with more fully developed social hierarchies come into being with the invention of agriculture, which implies more complex economies including markets and food storage facilities. These things require collective measures to operate and defend.
Hobbes believed that human beings in the state of nature would behave badly towards one another. Famously, he believed that such a state would lead to a "war of every man against every man" and make life "solitary, poor, nasty, brutish, and short." Hobbes's negative view of human character was shaped at least in part by the Christian doctrines of original sin and total depravity; the Christian tradition is generally at one with Hobbes in supporting the need for government and the innate, inherent, and inescapable sinfulness of human beings.
Hobbes's view was challenged in the eighteenth century by Jean-Jacques Rousseau, who affirmed instead that people in a state of nature would be born good; their bad habits are the products of civilization, and specifically social hierarchies, property, and markets. Rousseau's view underlines much of the Romantic period's political thinking, including the thought of Karl Marx.