Henna is a dye prepared from the plant Lawsonia inermis, also known as the henna tree, the mignonette tree, and the Egyptian privet, the sole species of the genus Lawsonia.
Henna can also refer to the temporary body art resulting from the staining of the skin from the dyes. After henna stains reach their peak color, they hold for a few days, then gradually wear off by way of exfoliation, typically within one to three weeks.
Yes it is allowed for men to apply henna on hands as tattoo provided that it does not have the name of Allah or any name of The 14 Infallibles according to obligatory precaution. Henna for men should not be like what is usually used by women.
Henna does not invalidate Wudu because it is just a color and not a layer on the skin which can prevent water from reaching the skin. Any color on the skin which has no layer but just color does not harm Wudu.