Enid Blyton's 'Six Cousins Again' (the sequel to Six Cousins at Mistletoe Farm) is the book to read if you want to understand the difference between 'high tea' and 'afternoon tea'.
Essentially, afternoon tea happens at about 4 o'clock and is a light meal of small sandwiches and cake, to tide you over until dinner, which wouldn't be eaten till about 8 o'clock.
High tea is a substantial meal eaten about six, instead of dinner, and would include things like sausage rolls, hard-boiled eggs, cheese, ham and other cold meats; large slices of cake etc.
A more modern use of 'tea' would be if you have 'dinner' in the middle of the day (i.e. a substantial hot meal) you might have 'tea' in the evening - a lighter meal combining elements of 'high tea' and 'afternoon tea'.
To further confuse the issue, some people refer to the midday meal as 'dinner' and the evening meal as 'tea', irrespective of what the meals feature - often a regional variation, but consider the term 'school dinners' as a common example.