Shake Lock is an application that lets you shake your phone to lock it. To use it, simply shake your phone in a left-to-right movement.
It's obviously a very bare application and shouldn't be expected to do more than what is stated.
In order for it to work, you need to enable it on Settings > Security > Device Administrators first. The reason for this is because applications cannot lock the screen without being a Device Administrator. If you already have it activated before allowing it as Device Administrator, you may need to set it to inactive then active again for the changes to take effect.