To use SSH on your mobile device you need to download a client. There are several different good quality clients available, such as Termius, JuiceSSH, and Connectbot. For the sake of this tutorial we'll use Termius, because it's a popular cross platform SSH client. But the principle is similar and applicable to other clients as well.
##1. Add your Raspberry Pi as a host
Download Termius from Google Play, if you haven't installed it already. Click to open the app.
The app should open showing ‘No Hosts’. To get started, you should tap the blue + button in the lower left corner. Then tap ‘New Host’.
Enter an alias; this could be Raspberry Pi, for example. Then enter the IP address under hostname. Fill in the username and password and hit the tick ✓ in the top right corner.
If you don't know the IP address, just type hostname -I in command line on the Raspberry Pi. See here for more ways of finding your IP address. The default login for Raspbian is pi with the password raspberry.
##2. Connect
After you’ve saved the new host, you’ll be sent back to the ‘Hosts’ screen. There you’ll find the new entry. Make sure your mobile device has WiFi turned on and connected to the same network as your Raspberry Pi.
Tap the new entry once, when the connection works you’ll see a security warning. Don’t worry: everything is fine. Click ‘Connect’. You’ll only see this warning the first time Termius connects to a Pi that it hasn’t seen before.
You should now see the Raspberry Pi prompt which will be identical to the one found on the Raspberry Pi itself.
pi@raspberrypi ~ $
You can type exit to close the terminal window.
If a dialogue saying Connection failed Connecting to 192.xxx.xxx.xxx port 22 appears, it is likely you have entered an incorrect IP address. Otherwise, WiFi on your mobile device might be turned off, the Raspberry Pi might be turned off, or it may be connected to a different network from your mobile device.
##3. Modify an entry, troubleshooting, and more
A connection might be unsuccessful for various reasons. The most likely is that your device or Raspberry Pi are not connected properly, SSH is disabled, there may be a typo in your code, or the IP address or credentials have changed. In the latter cases, you need to update the host.
To do this, go to the Hosts screen, and tap and hold the particular entry. In the top right corner, new functions will pop up. Tap the pencil icon. A new screen titled 'Edit Host' will pop up.