Raspberry Pi imager is an app that allows you to install an operating system on your Raspberry PI device.
What this app does is that it allows you to write the operating system directly on your sd card, this is the sd card that you will be using on your Raspberry Pi.
Raspberry Pi imager lets you choose which operating system you want to install, so you won't have to download the operating system manually.
In this tutorial, you will learn how to install Raspberry Pi OS on your Raspberry Pi device with Raspberry Pi imager on Ubuntu
Step 1 - Install Raspberry Pi Imager
Raspberry Pi imager is available on Ubuntu repositories, so to install Raspberry Pi imager on Ubuntu, you can run the command below
sudo apt install rpi-imager
Step 2 - Launch Raspberry Pi Imager
You can launch Raspberry Pi imager by running the command below
rpi-imager
Step 3 - Install Raspberry Pi OS
Installing an operating system to your Raspberry PI is simple.
First, you need to choose the operating system you want to install, to do this, simply click to "Choose OS"

You will see all operating systems, select the operating system you want to install

Next, you need to select micro sd card where you want to install the operating system.
Click to "Choose storage" button

and select the micro sd card

Once you have selected the OS, and selected the micro sd card, you can now click to "Write" button to install OS you selected on your micro sd card

Next, you will see the following prompt, click to "Yes" to confirm writing operating system on your micro sd card

Next, authenticate by entering sudo password

After confirming, the writing process will begin. Make sure to not close Raspberry Pi imager window or eject the micro sd card during the writing process.

Once OS has been installed on your micro sd card, you will see the following notification

You should be able to use the OS that you installed on micro sd card on your Raspberry Pi device.
Conclusion
In this tutorial, you learned how to install Raspberry Pi OS by using Raspberry Pi imager on Ubuntu.