Step 16 of 18 of the SMILES tutorial. Geometry around a double bond is written with directional bonds — / and \ — placed on the single bonds that flank it, never on the double bond itself. C/C=C/C is (E)-but-2-ene, the two methyls on opposite sides; C/C=C\C is the (Z) isomer. Because each slash describes the bond it sits on, one geometry has several spellings: F/C=C/F and F\C=C\F are both the E isomer. Leave the slashes out and the geometry is simply unspecified, which is not the same as "mixture".
It opens on C/C=C/C, 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.