For get detail ablout Signal and how Signal is better Alternative of Whatsapp Refer this.
There are two methods to install , 1) Using Snap 2) Via APT (using official Signal repository)
1) Install Signal Using Snap
For install using snap you can just go to Ubuntu Software (Software Center) Aft Search For Signal App And Install (Snap package format )
Alternatively you can just Open Terminal using CTRL + T and type below command
sudo snap install signal-desktop
2) Via APT (using official Signal repository)
You can use this method for all Debian / Ubuntu based Linux OS Like Linux Mint, elementary OS…….ETC
First get the GPG key for the official Signal repository and add it to the trusted keys of your APT package manager.
wget -O- https://updates.signal.org/desktop/apt/keys.asc | sudo apt-key add -
Now you can safely add the repository to your system.
echo "deb [arch=amd64] https://updates.signal.org/desktop/apt xenial main" | sudo tee -a /etc/apt/sources.list.d/signal-xenial.list
Now the Final Step.
sudo apt update && sudo apt install signal-desktop
Now you can search Signal app in application menu and start using your newly favourite Privacy Friendly Messaging app.