Step 13 of 18 of the SMILES tutorial. Put a mass number in front of the element symbol inside a bracket to specify an isotope. [13CH3]C(=O)O is acetic acid labelled with carbon-13 on the methyl — the sort of string you hand to an NMR or mass-spectrometry tool. Heavy water is [2H]O[2H], and some dialects also accept [D], but the numeric form is the portable one. An atom written without a mass number means natural isotopic abundance, not "unknown".
It opens on [13CH3]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.