what's the meaning of the callback function

Hello all,

I'm learning the EDMA3(used on omapl138)and I was confused when watching the code：

/* Callback function */
void callback(unsigned int tccNum, unsigned int status, void *appData)
{
(void)tccNum;
(void)appData;

if(EDMA3_XFER_COMPLETE == status)
{
/* Transfer completed successfully */
irqRaised = 1;
}

else if(EDMA3_CC_DMA_EVT_MISS == status)
{
/* Transfer resulted in DMA event miss error. */
irqRaised = -1;
}

else if(EDMA3_CC_QDMA_EVT_MISS == status)
{
/* Transfer resulted in QDMA event miss error. */
irqRaised = -2;
}
}

please tell me what's the meaning of this function and how to use it.