Step 8 of 18 of the SMILES tutorial. A molecule may have more than one ring open at a time, so ring bond numbers run from 0 to 9 — and %10, %11, … beyond that. C1CCC2CCCCC2C1 opens ring 1, opens ring 2 further along, closes 2, then closes 1: two fused rings sharing an edge, which is decalin. A number is released the moment its ring closes and can be reused, so C1CC1C1CC1 is two separate cyclopropanes joined by a bond. That reuse is why even a steroid needs only four digits.
It opens on C1CCC2CCCCC2C1, 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.