A content dictionary is the declaration of a collection of symbols, their names, descriptions, and rules. A published CD allows multiple parties to “speak the same OpenMath language”. An example of CD is setname1 which indicates the semantics of such symbols such as the set of natural numbers.
CDs are grouped in CD-groups. For example, the MathML CD group collects the ‘core CDs’, all content-dictionaries described in the OpenMath standard deemed compatible with the MathML3 Recommendation
The OpenMath Society curates a set of Content Dictionaries through its Editorial Board for Content Dictionaries. For the CD submission and curation process see the CD FAQ and the README on the CDs repository
Tools to validatate Cd and other OpenMath related files are described in the validation page, including an online validation tool
If you have trouble expressing yourself in terms of the existing CDs, maybe others had too, and you can find the answer in the OpenMath CDs FAQ