I was trying to create an enchantment and I had one of the square runes but I could not add it because it said 'needs a potency improvement' ?? What does that mean and how can you get a potency improvement ? (I couldn't find out)

thanks

I think that is a skill that you can take or acquire when you get higher level in enchanting. I am about 80% sure you get it automatically with more skill levels, but it could be purchased and I misremember.

You have to put actual skill points in crafting tree enchanting. Just have a look under crafting skills, cant miss. They are passives.

Have a look here.

You'll see Potency Improvement under the enchanting skill.

http://esohead.com/calculator/skills#zzVz5oE7D68B7r68zR7p68zf7c6

Basically for enchanting specifically there are two skills

One to increase potency
One to increase aspect

The essence runes can all be used from the start.

To use a (for example) Jejora Potency rune, you need to have Potency level 2
So you must spend a skill point to increase Potency in order to use it

If you wanted to also use a "rekuta" purple aspect rune, you would need Aspect level 3
So you must spend two skill points in Aspect in order to use that