Twitter Ads is an auction-based system, so your budget and bid determine the price of your campaign. Twitter will only charge advertisers when you acquire an action (based on your campaign objective).
If you are running a website click or conversion campaign, you will be charged for the clicks to your website that are acquired. All other actions and engagements (impressions, replies, and Retweets, for example) are free.
If you are running a Followers campaign, you will only be charged for the new follows you acquire through the campaign. All other actions and engagements (impressions, replies, and Retweets, for example) are free.
If you are running a Promoted Tweets campaign, you will only be charged for engagements on your Promoted Tweets. Impressions are free.
The daily budget is the maximum amount you want to spend per day. Your campaign will stop showing for the day once it reaches its daily budget; it will start again the following day. The total budget is the maximum amount you want to spend for the duration of your campaign. Once your campaign hits the total budget it will stop.
When setting up a campaign, advertisers have two options to choose from when determining how to set bids for their campaigns: automatic or maximum bidding. Automatic bidding enables Twitter to auto-optimise bids for a given campaign objective and budget on your behalf. Maximum bidding allows advertisers to manually select how much a lead, click, or engagement is worth to their business.
Remember that you can pause your campaign at any point, and review the results in your Twitter Ads dashboard.