A Y:Line is a line in the ircd configuration which tells the IRC Server what sort of clients and servers can connect to the server and gives certain properties of them:

Format:
Y:class number:ping frequency:connect frequency:max links:send queue length

connect frequency is applicable only to servers

In cyclone at least, there's also (in no particular order) a K:Line, Q:Line, C:Line, N:Line, X:Line, A:Line, M:Line, O:Line, I:Line, U:Line, L:Line, H:Line and a P:Line. There's also a G:Line