Welcome to Android Central! Who's your carrier? If you have a limited mobile data plan and have exceeded this month's data allotment, that means you won't be able to browse the web using mobile data, or use apps that require a data connection (unless you're connected to wi-fi). Or, if your carrier automatically charges you extra for going beyond your monthly limit, you'll have to pay more.
Calls and SMS texts don't use mobile data, so that won't be affected. MMS does use mobile data, but it may vary with the carrier of MMS counts towards your monthly mobile data limit -- best to contact your carrier and ask them.