Strings passed to SDL subroutines must be null-terminated (of type PChar in Pascal). This may be achieved either by declaring the variable to be passed as type PChar from the outset or by performing a typecast on a variable of type string. As its name suggests, a typecast (or sometimes just cast) converts one type of variable into another.
For example, the instruction PChar(MyString) converts the ordinary Pascal string MyString to a PChar string.
function SDL_LoadBMP(filename : PChar) : PSDL_Surface;