A Learning Rate is a discount applied to an ability depending on the Attributes of the character buying it. It is a total of three Attributes, though one Attribute is often used twice. Each Tree and Pillar will always have just one. This process is normally handled by the Character Generator, so players need not worry too much about the exact math.
They are typically described in the following format: 2*Dexterity + Will. This means that the ability in question receives a discount equal to twice the characters Dex plus their Will. If the character had a Dex of 14 and a Will of 12, the discount would be equal to 2*14 + 12, or 28 + 12, or 40. If this was an ability that normally cost 150 EP, it would then cost only 110 EP.