70- 300 means the focal lengths this lens is capable of... so when the lens is not extended it's on 70mm...when it's extended by rotating the ring it goes to the maximum value of 300mm
this is what focal length is:
this explains a bit better...
u see if the lens moves closer to the subject it will give a zoomed image...(like bird's eye)...which also means it's further away from the image sensor... so it would be 300mm...
so
70mm is the widest - you will get more in your picture.. like a big house or the image below
300mm is the telephoto end - you will be able to get "closer" to objects further away (Zoom/ a bird far away on a branch of tree)
hope this helps guys...