Your water heater will try an ignite cycle three times - I think it is three - between light attempts, the red light will come on briefly. If after three cycles, the heater still will not stay lit, then the red light will come on steady and the system is "locked out".
To try again, you need to switch the heater off for a couple of minutes and then you can try again.
If it won't light after a few attempts like this, then there is a problem that must be resolved.
If the heater has not been used on gas for a while, it could take a few cycles to light due to air that needs to be purged out.
Brian & Connie Mitchell
2005 Classic 30'
Hensley Arrow / Centramatics
2008 GMC Sierra SLT 2500HD,4x4,Crew Cab, Diesel, Leer cap.