DMP stands for Data Management Platform, and per Wikipedia, it is a technology platform used for collecting and managing data, mainly for digital marketing purposes. To put it simply, DMP is a collection of data about user attributes.
As you might expect a DMP may contain more than millions of data. The DMP I worked on contained nearly a billion. Open source toolings such as Elasticsearch provided a good average baseline for data analytics, and reasonable response time for general-purpose querying.