As a lender, when you provide liquidity to a pool, you will receive cpTokens relevant to that specific pool in return.
cpTokens have three main characteristics:
They represent the amount of liquidity that has been supplied to a specific pool
They accrue the interest rate for that pool on every block
They represent the risk profile of the pool borrower
cpTokens can be redeemed at anytime, subject to liquidity (see Withdrawing Liquidity for more information).
cpTokens are the building blocks for a broader system of tokenized credit and risk management. New features and products will be released which will give lenders (cpToken holders) more sophisticated optionality in terms of managing and hedging risks (see Roadmap for more information).