Contour lines indicate incline and decline in terrain. Slopes can affect the speed you will walk and how quickly you will get tired. You are looking out for orange lines with numbers by them. All along the '800 line' you are at 800m but if you walk of the line you will be either climbing of walking down a slope.
Contour lines are a way of representing to 3D maps on a 2D piece of paper.
This is how they are mapped. You can see that gradual slopes produce lines with wide spaces where steep slopes priduce lines close together. Imagine a very very steep cliff, the lines will be very close and you would not be able to walk over it. You won't find a footpath up there anyway.
Here are the steep and gradual slopes
A stream will always follow the valley down the hill...
A peak looks like this...
When planning your route, aviod steep hills and peaks, unless you particularly want to see a view or achieve a challenge.