Spend limits define the maximum amount of money that can be billed for a specific Campaign for a given renewal period. This limit will be applied to all ad sets that are grouped under the campaign.
You can set up a spend limit when creating a campaign.
You can update a spend limit by editing a campaign.
At this stage, this is optional.
However, if you want to control your spending, you can set a spend limit and a renewal frequency (daily, monthly, lifetime).