What is LibX11 ? RHEL neewbiee seeking help with definitions and concepts

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The title says it all. I guess one of my problems is understanding the operational concept behind Repositories etc..

Can someone explain what is a repository in lame language?
IS there others beside Redhat to get and install software>?
How I program the Add software tool to add these cloud like sites to add software not necesarilly from RED HAt?

Thanks in advance.