Step 6 of 18 of the SMILES tutorial. Lowercase letters mark aromatic atoms, so c1ccccc1 is benzene: six aromatic carbons closed into a ring, with no bond symbol anywhere. The same molecule can also be written with explicit alternating single and double bonds — the Kekulé form C1=CC=CC=C1 — and every toolkit will confirm the two strings describe one and the same molecule. The lowercase form is shorter and says "this ring is aromatic" without committing to a particular arrangement of double bonds. Paste the Kekulé form into the box and compare the depictions.
It opens on c1ccccc1, 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.