Many funders and journals now require DMPs. The 2015 HKU Policy on the Management of Research Records writes in paragraph 8, "[HKU] Researchers are responsible for: [..] developing and documenting clear procedures for the collection, storage, use, re-use, access and retention or destruction of the research data and records associated with their research. This include, where appropriate, defining protocols and responsibilities in a joint or multi-institution collaborative project. This information should be incorporated, where appropriate, in a research data management plan;"
A DMP is a digital document in which you describe what data you will collect during your research project, how you are going to store and manage the data during the project, and what will happen to the data after the project is finished. The nature of your research project, data to be collected, and the requirements you need to fulfill will determine how you write your DMP.