A journal database is a collection of articles from a range of journals relevant to a broad subject area. This collection can then be searched to identify articles that match your own subject criteria.
There are many databases available covering all aspects of health, such as Medicine, Nursing, Midwifery, Allied Health, Mental Health etc. as well as broader databases that look at subjects relevant to the social sciences and sciences generally. Which database(s) you choose to use will depend on your own subject of interest. For example, you may wish to search for information about the uptake of healthcare by families in urban areas, and in this case a social science database may be as useful as a more clinical nursing database.
The major databases relevant to nursing are CINAHL, standing for the Cumulative Index of Allied and Health Literature and the British Nursing Index, known as BNI.