tl;dr
It is safe to ignore this warning.
More Detail
Each Android application contains a “manifest” which contains
information about the application. This includes things like which
version of Android the application is designed for and other relevant
information.
MIT App Inventor automatically creates the manifest when you package
your application.
One of the items included in the manifest is a security profile which
contains information about which URLs (aka web links) an application can
connect to. Google has been pushing the use of secure http (aka SSL or
https). The warning you see is Google Play saying that your packaged app
has a security profile that permits insecure (unencrypted) web use. We
put that in the manifest because we (MIT) do not want to restrict which
sites your application may connect to.
However, as the author of the application, you can decide whether or not
to connect to encrypted or unencrypted websites.
We are discussing internally whether to provide a way to tell MIT App
Inventor to build apps with a more secure profile for those people to
use if they know that their application does not need to connect to
unencrypted sites.