High tea is a British concept that was originally about hunters returning too late for a regular teatime, typically between 4-5 p.m. As they would come in cold, probably wet, and hungry, they didn't always want to wait until a formal dinner time (usually 7.30 p.m. or even 8.30 p.m.) to munch on something.
It was also the name of a meal eaten by poorer people who couldn't take a break for teatime and had to wait longer to have their afternoon tea, meaning they needed something a little more substantial. In Scotland, it is more like a regular afternoon tea, but instead of just typical teatime fare, it includes a savory item or too like a cheesy pastry, to fill you up against the damp and cold. It was so popular in England that many people from working-class backgrounds still refer to their evening meal as "tea."
High tea is thus a glorious combination of "low" tea - that is, the kind you eat on comfy chairs or, weather permitting, in the garden - and dinner. It involves a delicious pot of hot tea to heat your chilly insides, as well as teacakes such as Swiss roll and French fancies, and perhaps a couple of scones and some sandwiches. What makes it really amazing is the way it also includes something more hearty, like a meaty pastry, Scotch eggs, or a slice of meat pie.
Time to nosh on your own high tea, and we'll see how hard you are to please!