As the name suggests, this indicator tracks the amount of tokens issued on a daily basis. This is a proxy to the inflation rate for the asset, but does not capture the full picture since it does not account for tokens being removed from circulation.
Issuance is helpful to track the amount of new supply entering the market. Under proof of work, this is received by miners in what is known as the block reward. As for proof of stake, its issuance is usually received by stakers.