皇 (huáng) is a bound form (cannot stand alone) that carries the meaning of 'emperor, imperial, royal'. You'll find it as the first component in key words like 皇帝 (emperor), 皇后 (empress/queen), 皇宫 (imperial palace), and 皇室 (royal family). Note that the dictionary sense 'uneasy; anxious' actually belongs to a different character 惶 — 皇 itself does not have that meaning. As a bound form, it always attaches to a following character to form a noun or adjective.