Immersion Heater is a Electrical `Direct to water` heater for a Hot Water Cylinder/Tank - when this is used it heats the water - often used as a `back-up` when for example the other source of heating the Cylinder/Tank fails, although in the U.K. this can be the `Primary` source of heating the hot water- far from economical! [ don`t know if used much in U.S.?]
The reason that the `Trip` switch/breaker turns off again - Instantly upon being switched on is because it is `doing its job` in protecting you - the IMMERSION HEATER has failed and caused an electrical fault - it MUST be REPLACED with a new one.
Please note - do not attempt to keep the `Trip` switch/Breaker from turning off by any temporary means - `Energising` the Immersion Heater for ANY reason could cause a `LIFE THREATENING` situation!!