烙铁 is a concrete tool noun — either an electric soldering iron used in electronics and light metalwork, or a branding iron (heated and pressed onto surfaces to mark wood, leather, or livestock). The English gloss 'flat soldering/branding iron; iron' is fine as a translation, but learners will benefit from knowing that the most common everyday use today is the soldering iron (焊接工具), while the branding-iron sense appears in historical or craft contexts. The pinyin here is just lào (the 铁 character is not toned separately because the word is a single compound with no space).