ΔH represents enthalpy change during a reaction while ΔH° represents standard enthalpy change (at 1 atm pressure and specified temperature).
Enthalpy (H): The heat content of a system at constant pressure.
A process that releases heat causing an increase in temperature of its surroundings; has negative ΔH value.
A process that absorbs heat causing a decrease in temperature of its surroundings; has positive ΔH value.