>>149731930A sigil is a magical thing. A basic version is to write down your intention as a sentence, then cut out all the vowels, and then all the repeating letters, then take what's left and overlap it to make a symbol.
Then you charge that symbol with power, giving yourself some kind of big rush is a good way. Morrison on one occasion charged it by going bungee jumping.
Then you burn the piece of paper with the charged Sigil.
So a Sigil is a purpose made, magically charged symbol.
A HyperSigil is a Sigil squared, a Sigil made of Sigils.
This is what The Invisibles was, and I think his Batman run as well.
What was the objective? Did it work?
It remains to be seen by anyone but Morrison, but some people have theories and suspicions.