Axial force is the compression or tension force acting in a member. If the axial force acts through the centroid of the member it is called concentric loading. If the force is not acting through the centroid it’s called eccentric loading. Eccentric loading produces a moment in the beam as a result of the load being a distance away from the centroid.
An interesting example of axial loading can be seen in the image to the right. Imagine a pogo stick person who is trying to jump with only one foot, what would happen? Well they wouldn’t stay upright. The ideal way to jump on a pogo stick is to ensure that the force is acting concentric to the pogo sticks centroid. If you are off by even a bit the moment will cause the pogo stick to react and you won’t jump straight.