Advertisers can use limits in their campaigns - you can find them in the "additional" tab while creating campaign. All of the limits are set on the UTC time ( the same as statistics).
All of the campaigns can be limited by money or impressions - you can choose the preferable way in the limits type.
In the first section you can set up limits by hours, days and determine the total amount of impressions or money that can be spent on this campaign.
In the second section you can set up the amount of impressions or money you want spend by specific days and hours.
For example, if you put 0 between 00:00-01:00 - it means that you won't get any impressions every Monday from 00:00 to 01:00. And if you use calendar limits, but leave the fields empty - you will get all of the available traffic this hours. Also you can see 1000 on the screenshot: it means, that you'll get 1000 impressions this hour. Please note: don't forget to check the types of limits while creating campaign.
It is not necessary to fill calendar limits. It is the additional function that allows you to limit your traffic by specific hours and days.