Take your time, there's no reason to act hastily. The command is safe, I'll let others elaborate, most linux users use it all the time. Here are a few explanations. It will sometimes fix issues to update and then upgrade the app lists.
http://askubuntu.com/questions/22234...-get-update-do
http://askubuntu.com/questions/94102...te-and-upgrade
http://askubuntu.com/questions/22234...-get-update-do
http://askubuntu.com/questions/94102...te-and-upgrade
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