Which WH you have depends on how light is wired
Light is a 'FAULT' light
Comes on if:
Propane fails to light off and PROVE it lit.
Usually 3 tries to light & prove then WH locks out and fault light comes on.
(ATWOOD & SUBURBAN BRANDS)
Fault light can also be illuminated if HIGH Temp T-stat opens/trips either on electric or propane (ATWOOD BRAND)
Light can also come on initially when propane 1st turned on and then goes out when flame lights & proves........will then come back on if fault happens (SUBURBAN BRAND)
Both electric or propane will only come back on after water temp has cooled below 110*F (ATWOOD) or 100*F (SUBURBAN)
Fault Light when illuminated (and lock out of WH) has to be reset by turning ON/OFF Switch OFF then back ON
Which BRAND & MODEL do you have?