To maximise your earnings, the default settings on your account are for your capital repayments (the money that comes back when a loan is repaid) to be automatically re-invested, and your interest payment to be automatically invested in new loans.
Having auto re-investment set up means that when a loan is repaid, your money can be lent out again straight away if there are loans available. By having your interest automatically invested for you, you earn interest on your interest, this is called compound interest.
Automatic re-investment is a great way to grow your investment, but it means that you don't have the same access to your money as you would if your capital repayments and interest were set to go to your available balance. You would need to sell the investment if you wanted to withdraw the interest payment.
Good to know
You can change your auto re-invest/invest settings any time by signing in to your account and going to My Account > Manage Income.