A database is a searchable electronic collection of information. Databases contain thousands of articles published in hundreds of different journals, allowing you to find relevant articles faster than you would by searching individual journals one-by-one.
Some databases are full text meaning they provide the complete text of articles. Some databases will only provide records of articles; records include information about the article: title, author, journal it was published in, publication date, summary, etc. Many databases provide a combination of records and full text.
Additional information about databases:
Depending on the vendor, databases might look different. However, they will often have similar functionality, like limiting to only peer-reviewed journals, publication date, etc.