Even though nowadays we don't have a Beit HaMikdash, and so the Kohanim can't do the work of bringing korbanot (sacrifices), Kohanim today still have special status. One of the ways that we can still experience this is in synagogue on the holidays of Pesach, Shavuot, and Sukkot, when the Kohanim bless the congregation. The special blessing they say is called Birkat HaKohanim. (In Israel, Birkat HaKohanim is said every day, not just on Yom Tov.)
At the end of the Mussaf ("additional") prayer, the chazzan (cantor) calls, "Kohanim!" All of the Kohanim come forward and stand facing the congregation. They hold up their hands in a special shape - it's a little like the secret handshake of the Kohanim, and it's used only by them, and only during Birkat HaKohanim.
The "windows" between their fingers let the "light" in.
