What does "use cellular data for" mean?

For apps that require a data connection, if you enable that and you are not within range of a wifi node to use, the app will use cellular data which will count towards your cell plan's data usage. If you turn that OFF, and there is no wifi available, the app will not work, but you also won't use any cellular data.

so in the iTunes and App store settings, if you want to limit the stores to only use wifi, turn the "Use Cellular Data" option OFF (e.g. to avoid big downloads over cellular).

It means that you are enabling/disabling your device from using cellular data (i.e., your cell service provider's internet access). If it is disabled completely, then you will only be able to connect to the internet via Wi-Fi.