The Priestly Blessing is both a piece of Torah and a piece of liturgy. In the Torah, it's Numbers 6:24-26, and the text goes:
May the Lord bless you and keep you; May the Lord shine His countenance toward you and be gracious to you; May the Lord lift up His face toward you and give you peace
Liturgically, it's chanted on certain occasions by any cohanim who happen to be present, and while they do it, they hold up their hands in a Vulcan salute (the two are actually related, although the Vulcan thing is from the Jewish thing and not the other way round).
See Wikipedia for more on this.
OK, on to the scribal bit.
Each of the verses in the Priestly Blessing is separated by a white space. The white spaces are part of the text of the Torah; you have to have them in the right places relative to the text, but their positions on the page aren't prescribed, so they can be bigger or smaller depending on how you want to space the text out.
At some point, some scribe realised that if you're a bit canny with how you place the white spaces, you can arrange them so that the white spaces around the words of the Blessing make the shape the Priests' fingers are in when they say it:
PS - the hand in the picture is not touching the text. It is about a centimetre above it.