1) Redshift is a result of the space between the Earth and the galaxies expanding. This expansion stretches out the light waves during their journey to us, shifting them towards the red end of the spectrum. The more red-shifted the light from a galaxy is, the faster the galaxy is moving away from Earth.
2) The theory states that about 13.7 billion years ago all the matter in the Universe was concentrated into a single incredibly tiny point. This began to enlarge rapidly in a hot explosion, and it is still expanding today.
As objects move away from an observer the wavelengths are stretched, creating longer wavelengths. The longest wavelengths of visible light are the red wavelengths. As objects move toward an observer the wavelengths are shorten.It is this observation of the red shift that gave rise to the theory of the Big Bang.