Explore was born out of Moments, which was Twitter's push to highlight the biggest news and trending topics across the platform. Now, this tab will have Promoted Trend Spotlight ads, which will consist of a large visual banner above the regular explore content. These will appear the first two times per day you visit the tab, and then disappear after that.
TechCrunch reports that the first Explore tab takeover will be from Disney. The company will be promoting the upcoming movie Christopher Robin.
No one likes inserted advertising into organic experiences, but Twitter is free to use. It's understandable that the company is searching for additional (and better) ways to monetize the service. The last quarter of 2017 was the first time that Twitter reported profitability; that continued in Q1 of 2018. Let's just hope these ads doesn't disrupt the user experience too much.