Step 15 of 18 of the SMILES tutorial. A tetrahedral stereocenter is marked with a chirality tag inside its bracket. Look from the first neighbour in writing order towards the centre: @ means the other three, in writing order, run anticlockwise, and @@ means clockwise. C[C@H](N)C(=O)O is L-alanine; change the tag to C[C@@H](N)C(=O)O and you have D-alanine. Because the tag is relative to writing order, the same molecule can be @ in one string and @@ in another — N[C@@H](C)C(=O)O is L-alanine as well.
It opens on C[C@H](N)C(=O)O, which you can edit in place: the structure follows every keystroke, and so does the notation when you draw instead.
Convert between chemical structures and SMILES in both directions, search a list of structures by substructure or SMARTS, and learn the notation step by step. Everything runs in your browser — no structure you open ever leaves your machine — so the tool needs JavaScript.