Uninstall component means to remove the component from the Joomla site. If you don't want to use a component in your Joomla site then you have to uninstall it.
Disable component means you are not interested in the component right now but may be in future you want to use it, in such situation you have to disable it.
In short Joomla just restricts the disabled components from showing the output. Whenever you want to use the disabled component you just need to enable it.
Doing this doesn't require coding, there is option for this at the backend. Here is the image. When i wanted to disable the component first time, then i was wondering that i need to make the change in coding.
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