five couple sets. Means that you are in couples, facing your partner, with "men" on one side facing "women" on the other. You need 5 couples in a set, if there's extra couples, they'll need to start a new set.
Typically the phrases of the music are described as 'A' parts and 'B' parts, the description of the dance refers to the four phrases. A1, A2, B1 and B2...
Here, you'll find it easy to recognise the A1 and A2 parts, the two 'B' parts can quite happily run together.
Join hands in line and... That is the five people on the "men's" side hold hands and the five people on the "ladies'" side hold hands
When back at the top of the set the first couple face each other and all the other people face up.
All the men are on one side and all the women on the other and the pattern is a sort-of up-side-down horseshoe.
If you are second, third, fourth (etc) couple dancing up the set, you will be doing the same, right hand, left hand, right hand etc. You are dancing along the line of that up-side-down horseshoe (or according to Mr Pott's description, up one trouser leg and down the other)