They've banned US transactions with the company. This means TikTok can no longer publish to Play Store/App Store. Android users could sideload an APK but that's still a huge blow. Lastly a social media app operating in the US that can't sell ads to US marketers is pointless.

As an alternative, Trump proposed letting a US company buy TikTok to get around the ban. As you'd expect, TikTok's investors are looking for a sale so they don't lose their investment.