Google Japanese Input/Simeji warning about inserted data
Basically, it's saying that the app could potentially steal your information, as it's a keyboard and therefore processes what you type. For instance, if you typed your password in and it was malicious, it could potentially record that (and everything else you type). Of course, you could trust Google wouldn't do that...Android is simply reminding you of the risk. 

Further reading: The FAQ for the Hacker's Keyboard

The warning can be found in the Android source (see line ~2687).