General installation steps on Linux
Each distribution of Linux (including Raspbian) maintains a library of software in the form of "packages" and comes with a package manager to download, install, update and remove these packages. Many applications can be installed by installing a single package while others require a list of packages. You will usually find what packages are required for an application by using the keyword "install" and the application name in a search engine.
Raspbian uses the Advanced Package Tool to install applications. For example, to install the gedit text editor, you are recommended to refresh the list of available packages using:
sudo apt-get update
"sudo" means runs as super user and you will be prompted for the superuser password unless you have run sudo recently
"apt-get" is an Advanced Package Tool command.
"update" gets the updated list of packages. It does NOT update or install any software.
This step is optional, and can be skipped if you have refreshed the package list recently.
Install the gedit applaction using:
sudo apt-get install gedit
"install" installs the packages you list.
"gedit" is the single package you want installed.
The size of the packge(s) you want to install will be displayed, and you will be asked to confirm you want to continue. Just hit "y" and "ENTER". If you don't want to be prompted, you can add a "-y" to the end of the commnd like this:
sudo apt-get install gedit -y