What is a daily budget in youtube and how does it work

Your daily budget is the amount that you're willing to spend on your entire Promoted Video account each day. At this time, this programme applies an overall budget to all promotions within your account, rather than a budget per promotion.
We'll display your promotions as often as possible, while staying within your daily budget. When we reach your budget, your promotions will typically stop being shown for that day. Traffic can fluctuate day to day, so that some days users might not click your promotion much, while on other days they may receive a lot of clicks. To help make up for those low traffic days, on higher traffic days, we'll carry over up to 20% of your unused budget from those previous days. However, at the end of the month, we'll never charge you more than your daily budget, times the number of days in that month.
For example, let's say your budget is USD10 per day in a month with 30 days in the billing period. On some days of the month you may receive up to USD12 in clicks, while on other days you could receive far less than the USD10 daily budget. At the end of the month, regardless of how your traffic and clicks fluctuated, we'll charge you no more than USD300 for the calendar month.
To edit your daily budget, follow the steps below:

1. Sign in to your account at https://ads.youtube.com
2. Click "edit" in the Budget section on the left-hand side of your Promoted Videos dashboard.
3. Enter a new amount for Max. daily (average).
4. Click "Submit budget".