So Grindr is a gay dating/hookup mobile app, the chats/DMs for which are stored entirely on your device (unless you opt to enable Google Drive backups).
As such you can lose your chats if you delete and reinstall the app, resulting in the note of "lost chats" on many users' profiles.

Today I saw somebody with a screenshot of the Urban Dictionary entry for "lost chats" and I found it sometime entertaining (if a little... Crude/insensitive?) https://www.urbandictionary.com/define.php?term=lost+chats