Local-first allows users to enjoy all the benefits cloud apps bring (like seamless real-time collaboration, syncing, auto backups) while keeping the data ownership aspect from traditional desktop software.

An ejectable app allows users to "eject" at any time:

• get a zip of all the data
• download the binary to run on a backend sync server or locally
• able to import to the app again

and a comment from Sebsauvage:

• the data format must be open and free

(via https://sebsauvage.net/links/?b3epNQ)