That is the channel level in mIRC, Just click yes and continue with what it needs to do to connect to the server ;)
Just so you know the levels are as follows with the highest being at the top.
~ = 0wner of the channel
& = AOP
@ = SOP
% = HOP
+ = Voice
Bah! I always get @ and & mixed up ;D
/WSV.autoloadpickup