At the beginning of the film Scar said to Mufasa'Perhaps you should'nt turn your back on me. What did he mean by that?.

That if Mufasa wasn't careful, he (Scar) just might stab his back. Hence, murder his child and even Mufasa himself. It is hinted in Scar's words to Mufasa before that line that he (Scar) detestes not only Mufasa's son existing but also how high and mighty Mufasa acts while Scar himself is bitter about never getting to be a king anyway.

This has two meanings
(1) Scar may stab him in the back
(2) Scar feels as if Mufasa abandoned him/shut him out