Nobody knows what the ankh depicts. This is the honest position and it is worth stating clearly, because the confident explanations are numerous and none of them is established.
Alan Gardiner, whose sign list remains the standard reference, suggested a sandal strap, the loop passing over the foot and the bar across it. Others have proposed a knotted girdle or sash, a stylised composite of the was and djed, a mirror, a sun rising over the horizon, and various anatomical readings. Each explanation accounts for some depictions and not others. The sign is attested long before any Egyptian text attempts to say what it shows, and no Egyptian text ever does.
Its afterlife is unusually eventful. Coptic Christians in Egypt adopted it as the crux ansata, the handled cross, and used it in manuscripts and church decoration, a straightforward reuse in which a native sign for life was read as compatible with a new claim about life. From there it entered Christian iconographic vocabulary in Egypt and Nubia.
Its modern circulation begins with the European rediscovery of Egypt and accelerates through nineteenth-century occultism, which had views about Egypt that Egypt would not have recognised. In the twentieth century it was taken up by Kemetic reconstructionist religion, by Black nationalist and Afrocentric movements as a sign of African antiquity, and, by an unrelated route, by the gothic subculture. None of these uses is illegitimate, but none of them is Egyptian, and the meanings assigned in each are supplied by the users.