Tell me the problem with package package ltpackagegt having no installation candidate?

What does the above error mean? What is an installation candidate?

I was trying to do

sudo apt-get install munin-memcached

and I get this message.

Reading package lists... Done
Building dependency tree
Reading state information... Done
Package munin-memcached is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package munin-memcached has no installation candidate

I googled for the problem and someone said to do a apt-get upgrade but it still did not solve my problem.

This occurs when you try to install a package about which apt has no idea When you add software sources, you need to do an apt-get update so that your system APT's database is updated with all the packages on the repositories listed in the software sources lists.

Then when you try to install any package, apt checks the package name in it's database, finds it and checks the name of the repo from where it got it. It then downloads the package from that repository

This means of all the repositories listed in your software sources, none of them have the package named munin-memcached . I checked again and it's not available in the official ubuntu repos

After a bit of googling I found that it is available in this PPA: chris-lea/munin-plugins

Add this ppa and install the packages on your own risk I do not know this person

To add the ppa, issue the below command.

sudo add-apt-repository ppa:chris-lea/munin-plugins

To install package munin-memcached , issue the below command.

sudo apt-get update && sudo apt-get install munin-memcached