The lines that make up your character describe the shape, form, texture and positioning of all their different parts. To keep your character from looking stiff, you try to use a line of action that generally speaking, runs from the neck, through the spine and usually out one or both legs to the heel of the foot. This is called the "Primary Line of Action" in your pose.
There are secondary lines of action that run throughout the body as shown in the example above. It's not always necessary to have these lines, but it does create more of a "flow" to the design. It also helps with the animation of the character to create direction to the movements and allow for overlapping action to look more natural.
During the 1990's there was a revolution in animation design that went to more graphic shapes in the characters using very primary shapes such as circles, ovals, squares, rectangles and triangles. The use of straights against curves created very bold designs in shows like Dexter's Laboratory, Samurai Jack, The Power Puff Girls and many others.
However, if you take a close look at the drawings above, you will still find the principle of the lines of action in each one.
Study animation model sheet to find these lines of action and then begin to incorporate them into your own drawings.