Suppose the flag of a package is "pi". I learnt that the "i" stands for "i - the package will be installed." What's the meaning of "will be installed", please?
With aptitude you first mark what you want to have done, and then you proceed to do it. For example you might press the find a package you want to install and then press the "+" key. Assuming the package isn't currently installed the first two flags then will be pi, meaning the package is either purged or never installed, and is to be installed.
To actually install it, you press the g key and aptitude will proceed to install it.
If you change your mind before you do that you can "Cancel pending actions" in the Actions menu and anything you had marked for installation or update will be unmarked. There are menus and reminders about what keys do what on the top of the aptitude program's display.
The synaptics program has a similar approach but has a real gui and may be easier to use.
The other approach is to use the ubuntu software center to install software and the update-manager to keep it up to date. They do things more immediately.