That's what costumed revelers were shouting when I went downtown on Saturday. I actually went to shop but ended up watching some of the Karneval parade anyway because the streets were just full of people and getting anywhere was difficult.
"Helau!" is the standard Karneval 'battle cry' and originated in Düsseldorf. If you live in Cologne, however, you need to shout "Kölle Alaaf!" (Köln vor allem) - roughly "Cologne before all the rest!". In Rheinland-Pfalz you'll often hear "Narri Narro!" and in Mannheim they say "Ahoi!"
My kids went off to school today for a bit of fun looking like this