Basically, a credit is a unit of cost used when sending or receiving text and voice messages.
Credits work in two ways.
Here is current pricing information
If you are unsure now many credits a text will cost you, when you are in the message box, click the "?" and it will open up your message details - including how many characters and credits your message will use.
Anywhere you type a message will have a similar "ticker" to let you know how many credits and characters your message is using.
The voice broadcast also lets you know how many credits it will use before sending it: