You know what speed is, right? It’s a certain distance covered within a certain time (let’s say in metres per second). So, if you’re travelling at 10 m/s, we know how fast you’re going, but we don’t know where you’re going. Speed is a scalar – it has magnitude, but no direction. A vector (as opposed to a scalar) has not just magnitude, but also direction. If you express your 10 m/s with an arrow for direction, it is no longer speed (a scalar), it is velocity (a vector).
Like velocity, force is a vector. It has not just magnitude, but also direction. The forces acting on your aircraft can be illustrated by drawing a diagram with arrows pointing in each direction toward which the forces are acting. For example, gravity is a force acting downward from the aircraft. So, draw an arrow downward to represent that force. There will be other forces acting on it as well – you’ll have to figure out what they are, and add them to your diagram.
Often, the arrows are drawn to (roughly) the scale of the magnitude of the forces. So, bigger forces get bigger arrows.
Does this make it a bit clearer? I’m actually surprised that you’ve been asked to draw one if you don’t know what it is. Is it for a class? If so, at what level?