An English term used to refer to some Jewish festivals.
Only two festivals are considered "High Holy Days". They are:
Much more detail on the specific festivals is included in their own nodes. A former writeup said that Succot and Simchat Torah are also High Holy Days. While they are important festivals, and fall shortly after Yom Kippur, they are of a lesser level of importance, alongside Pesach and Shavuot.
The most commonly used Hebrew term for the High Holy Days is "Yamim Nora'im" which literally means "Days of Awe". On Rosh Hashanah and Yom Kippur, Jews believe we are judged by G-D for what we have done in the year that has just finished (Rosh Hashanah is the first day of the Jewish year), and G-D decides what sort of year we are going to have (the Unetaneh Tokef prayer spells this out very clearly).