Why exactly are images formed by lenses/mirrors? I just don't get the concept behind why a lens or a mirror forms a reproduction of the object at present. Is it to do with the object blocking parts of the light source? I just don't understand why an image of that object is produced.
The question is essentially trying to understand what causes the image to be produced from the object, in the first place.
 A: The reason why an object is visible is that each point of that object emits or diffuses light (reflect in all directions). Let's take one point "A" of that object, say one pixel of your screen, and consider the light field in an imaginary plane that we will move away from it.
When the plane is right on the screen, only the light from that particular point A reaches a particular point B on the screen. There's a one-to-one match, it's the Object.
As you slowly move the plane away from the screen, the light coming from point A creates a wider and wider spot on the observation plane. Since light from all the points of the object add up, you get a more and more blurry reproduction of the object. The light from each point of the screen flood the space with rays going in in all directions.

Now what a lens does is bend light rays with the remarkable property that the diverging rays originating from a point at a distance L in front of it will cross at a single point a distance L' behind it! 
And this happens for all the points of the object at that distance L. So if you move your observation plane to that distance, all the rays are sorted in space again to produce single spots. There is a one-to-one match between the Object plane and that plane, it is the **Image*.

By putting a detector, a camera CCD or film or your retina, at the image plane, one can "see" the object.
I hope it helps!
N.B.: Light rays propagate in straight line in a (homogeneous) medium. It only bend at the interface between two mediums of different refractive index
N.B.2: The distance L' where the rays recombine depends on its focal length f, which itself depend on the curvature of the surfaces of the lens and the index of the lens material. 
N.B.3: Everything your eyes see are not the object themselves, but their image by your eye lenses!
