"Development mode"

[py_lrnr]

I am new to python and I have come across the following command and its description: 

>Now to be able to run the project you will need to install it and its >dependencies. 

>python setup.py develop

I looked up what the 'develop' argument does and found:

>Extra commands:
>  develop           install package in 'development mode'

I searched for a description of 'development mode' but could not find a good description.

Can anyone (very briefly) explain to me, in a sentence or two:

what 'development mode' is? 
how 'development mode' differs from other 'modes'?
why/when I would use 'development mode'? 
what 'development mode' does or does not allow me to do?

Many thanks in advance.