The "concrete syntax" of a program, in our case of an expression, is the
  syntax the programmer use while writing it. The "abstract syntax" is a
  representation of what the concrete syntax means. An "abstract syntax tree"
  (AST for short) is a tree representation of the abstract syntax. A syntax tree
  representation as no more associativity and precedence problems.

  So after parsing the string "1*(2+x)*(y-10)" we get the AST:
    Ast.Times
      (Ast.Times
        (Ast.Int 1,
        Ast.Plus
          (Ast.Int 2,
          Ast.Ident "x")),
        Ast.Minus
          (Ast.Ident "y",
          Ast.Int 10))
  that is:
              *
             / \
            /   \
           *     -
          / \    | \
         1   +   y  10
            / \
           2   x
 Having an AST and a mapping from variable names to variable values you can
 easily compute the value of an expression with a recursive function.