At the beginning of the film Scar a dit to Mufasa'Perhaps toi should'nt turn your back on me. What did he mean par 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 amer 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