Install from phone storage
This setting allows the installation of a security certificate, such as a trusted Certificate Authority certificate, from a file stored on your phone's storage.
Security certificates stored on your device can be used to verify the authenticity of a server that you are connecting to using a secure connection.
You may need to install additional security certificates on your device if, for example, you need to make a secure connection to a corporate server and you need to verify its authenticity with a certificate signed by a server within your company. The certificate may have been installed by software provided by your company.
You should not normally need to do this to connect to most networks or secure servers on the wider internet.
For more information you may read Trusted credentials.