DLP stands for Data Loss Prevention. You wouldn’t want sensitive information, such as social security numbers, credit card numbers, health records, etc. to be easily transferred across or out of the network.
DLP can be used to stop this sensitive data from being dispersed. For example, if you send an email containing social security numbers, a DLP policy can stop this email from being sent because it detects sensitive information. Email is just one example. There are many different ways that data can be leaked out of the network, so DLP may require multiple different solutions to plug all the holes.