It refers to the amount a user is willing to spend on certain Adwords campaign daily.

As an advertiser there is an upper limit you might have while spending on Adwords. These limits when applicable on a daily basis are often referred to as daily budget. For a given daily budget, Adwords will try displaying your ads as much as possible. When the budget gets crossed, ads will no longer be displayed for that given day. Depending upon your daily budget Adwords system can deliver as much as 20% more ads for a given day. This procedure ensures you will make up for those days when your daily budget hasn’t been crossed and it falls well within the billing period