Say you have a plot of an incident, for example the position of a particle. The partcle starts at one point, and takes a journey through space, and ends up somewhere else. If the particle had started at almost the same spot, but not quite, the particle will typically end up at almost the same spot, but not quite, taking a similar route. Or if the particle start at the same point, but some of the other initial conditions vairy a little,  like the initial velocity, angle or acceleration. Or any other initial value you might think of. A very small alteration in the initial values, would change where it ends up, but not by a lot.
The definition of chaos is where the particle ends up in a COMPLETELY different place, and uses a completely different  path to get there, after altering the initial condittions in a VERY small degree. If we plot the position vs. the velocity, or the angle vs. the angular velocity, we will most often get an equally chaotic plot, as the position vs. time.  But once there was a man, who, when ploting, got a systematic plot of position vs. velocity, when time vs. position was chaotic. The plot he got looked somewhat like a butterfly. The butterfly effect is a term one use,  for how small changes can have great impact on the further evolvment of things. For example can a butterfly in Greece determine wheater you, here in Norway live or die, by either flaping its wings or not. In one case it will cause a chain reaction evolving into your death, in the other case it will not. This is a very interesting subject within timetravel, among other things. If you google pictures of  ‘lorenz atractor’, you will find many beautiful pictuers. you can also wiki Lorenz atractor, the butterfly effect, and chaos theory.