Applications restore or install from external sd card
This is due to the fact that by default non google-play applications are blocked by default in Android, as a security measure.
To allow these to be installed go to (this may vary in wording across Android Versions)
Settings &gt; Security &gt; Unknown Sources this allows things to be installed that are not form the known (Google Play Store) source.
You can, in later versions, set only the inital install to process or allow any further ones to install.
In android it is similar to this (slight variety in different versions)
http://www.fannsoftware.com/img/UnknownSrc.png
On the Kindle it is slightly different:

(source: bestvpn.com)