I don't know much about the specifics of what Twitter are selling, but there's two possibilities:
1. They're just selling raw feeds of data that people can pay for and analyse as they wish. This is something they definitely are doing, it's called the "Twitter Firehose", and isn't cheap to get access to.
2. They're selling their own, presumably anonymous, analysis of users for external advertising purposes. For example if I've tweeted about wanting to buy a car, maybe some car company's agency wants to buy data that will allow them to (re)target me outside of Twitter - I put (re) in brackets as it's the same concept as retargeting, but technically just targeting as I wouldn't (necessarily) have already visited that car seller's website. I've no idea if Twitter do anything like this, and I'm not entirely sure how it would work from a technical point of view, I've done marketing based around very targeted data, but only ever through companies that specialise on figuring out who to show adverts to, so I've never had to worry about how they were doing it.
1. They're just selling raw feeds of data that people can pay for and analyse as they wish. This is something they definitely are doing, it's called the "Twitter Firehose", and isn't cheap to get access to.
2. They're selling their own, presumably anonymous, analysis of users for external advertising purposes. For example if I've tweeted about wanting to buy a car, maybe some car company's agency wants to buy data that will allow them to (re)target me outside of Twitter - I put (re) in brackets as it's the same concept as retargeting, but technically just targeting as I wouldn't (necessarily) have already visited that car seller's website. I've no idea if Twitter do anything like this, and I'm not entirely sure how it would work from a technical point of view, I've done marketing based around very targeted data, but only ever through companies that specialise on figuring out who to show adverts to, so I've never had to worry about how they were doing it.