DTB is an abbreviation for Doop-, Trouw- en Begraafboeken [Baptismal, marriage, and burial books]. Sometimes, you see it as DTBL where the L stands for lidmaten [members].
You may come across the term in publications, finding aids, research guides, or source citations.
Check this article for more Dutch genealogical abbreviations.
Finally a clear definition of lidmaten. I searched and only found vague information, mostly about lists. Thank you very much Yvette.