A licence clarifies the terms of use for your data – disentangling what can otherwise be quite a complicated default legal position.
A licence is a formal statement issued by the holder of the rights to a particular work (e.g., a database or other dataset), permitting to use of the work in certain specified ways. You might, for example, specify that if your data is reused, you must be cited as the creator or that the data may be used for research and educational purposes but not in commercial contexts.
Commonly used open licenses for data include Creative Commons and Open Data Commons. Alternatively, if you control all rights to your dataset and wish to make it available for others to use without any restrictions, you might consider using a Creative Commons Open Data CC Zero public domain dedication and waiver.
The DCC’s guide, How to License Research Data, provides a useful overview.