I have seen some definition like, It is a quantity that is coming from the time translational invariance. or a quantity that should be conserved in some process. But I am not able to understand what exactly energy is? In STR mass even can convert into energy, that also make me so confused. So everything can be converted into energy. I conclude that everything is actually nothing but energy. But I find still there are lot of problem of thinking in that way. Please help!!