The DDIC or Data dictionary plays an huge tremendous fantastic great role in what SAP does. The ABAP Dictionary centrally describes and manages globally defined data definitions used in the system. It describes tables, table keys, but also the table fields down to possible values. Search help and lock object, number ranges, the list is pretty endless. These articles will look like they are not very related to each other, yet they all describe the great DDIC.

If DDIC is related to a single transaction (which it is not) - it would be the SE11 transaction. Tables and structures contain fields, a field is assigned a data element (or it is assigned to an internal data type) and the data element is described in a domain. The data element holds information on the description and label texts. The domain is about the values that the field should support, it in turn describes possible values or check tables, conversion routines (function modules) and such. But the lock object and Search help are also DDIC matters.