The 8 are English (best of Lang and lit but have to take both) and Maths, both double weighted. If lit not taken, then English only counts once. Then three EBacc subjects (Sciences, history, geography, MFL or ancient languages, computing), then three other subjects, which can be GCSEs or approved vocational subjects. The total point score across those subjects is divided by 10 to give the attainment 8 score per student. And the mean attainment 8 score across the cohort is the attainment 8 score for the school.
The progress 8 is taken by looking at the end of KS2 attainment of the student (fine levelled, so eg 4.6), and comparing their attainment 8 with every other student in the country with the same KS2 fine level. So if they are exactly the same as the average, that is a P8 score of 0. If they are on average 1 grade higher, P8 will be 1. Again the P8 scores for all the year 11s are averaged to give the school's P8 score.
The EBacc is a measure of how many students took, and achieved a particular grade (4 for good pass, 5 for strong pass) in English, Maths, 2 Sciences, History or Geography and a language. I would suspect school a doesn't insist on a language at GCSE.
High prior attainment means 5.0 or above for KS2. Middle prior attainment is 4.0 - 4.9.