It appears to be a definition.
Interest rate is (defined to be the) average prime lending rate [until] June 2010 and [then] base rate afterwards.
It is defining interest rate, which appears in the graphic. I have boxed in in purple.
I'm not entirely familiar with these financial terms, but "prime lending rate" is one thing, and "base rate" is another thing. I am not sure if "average" means "typical", or if "average prime lending rate" has a special meaning in finances.
Here is some info I found on the two.
What is the 'Prime Rate'
The prime rate is the interest rate that commercial banks charge their most credit-worthy customers.
- (Banking & Finance) Brit the rate of interest used by individual commercial banks as a basis for their lending rates
- (Banking & Finance) informal Brit the rate at which the Bank of England lends to the discount houses, which effectively controls the interest rates charged throughout the banking system
So all together, the average prime lending rate is used as the interest rate until June 2010, and then base rate is used as the interest rate afterwards.