Spend Cap allows you to set the maximum amount of money the campaign can spend during its lifetime. You can edit the budget normally, Spend Cap just ensures that the campaign does not spend in total more than what you want. Spend Cap needs to be at least 100 USD in local currency.
With campaign spend cap you can set a limit, for example, a campaign with high daily budgets and then cap the spend with spend cap. In this way, there is no need to calculate the end budget.
How to enable the Campaign Spend Cap?
You can enable Spend Cap in the Campaign tab in campaign creation or for an already existing campaign in Campaign editing mode.
How to edit Spend Cap in running campaign?
You can do edits to the spend cap from Campaign view --> Edit campaign
Campaign tab --> Spend cap
See also: Account Spending Limit