Hi there, I'm trying to sheafify a constant presheaf on a site, I went to http://ncatlab.org/nlab/show/sheafification, but can't understand the notation in the equation for W (in the proof for existence)

W = {S(U_i) := lim... }.

Can anyone help me figure out the notation? With respect to what is the limit taken? I'm still trying to figure out the whole sheafification process but so far I have like 2 or 3 books on the subject and none of them has been exactly helpful or straightforward for my liking, I need this result for a proof I'm working on and am not exactly an expert in Topos theory, I just need this result, hope you guys can help.