Credits are the number of SMS messages allotted to your account, per the plan you make with your account executive. This does not count MMS messages or other features on your plan. You can view the transactions that affect credits by looking at the Usage Log within the Usage Tab.
Once your SMS credits are used, it will then use your monetary balance. MMS, social profiling, and carrier lookups are directly used from your balance.
It's important to keep in mind that all incoming and outgoing SMS are counted as a credit.
If you need more messages, you can add funds or call in to 1-855-MOBINITI