Whether we like it or not, many if not most of us developers either regularly work with databases or may have to work with one someday. And considering the amount of misuse and abuse in the wild, and the volume of database-related questions that come up every day, it's fair to say that there are certain concepts that developers should know - even if they don't design or work with databases today. So:

What are the important concepts that developers and other software professionals ought to know about databases?

Guidelines for Responses:

Keep your list short.
One concept per answer is best.

Be specific.
"Data modelling" may be an important skill, but what does that mean precisely?

Explain your rationale.
Why is your concept important? Don't just say "use indexes." Don't fall into "best practices." Convince your audience to go learn more.

Upvote answers you agree with.
Read other people's answers first. One high-ranked answer is a more effective statement than two low-ranked ones. If you have more to add, either add a comment or reference the original.

Don't downvote something just because it doesn't apply to you personally.
We all work in different domains. The objective here is to provide direction for database novices to gain a well-founded, well-rounded understanding of database design and database-driven development, not to compete for the title of most-important.