"Registering" is user's action - entering an entry, which contains some predefined set of information, into certain table. E.g. when you enter an info about a new project into projects table, you are registering a new project there, and only after that you can use this new project in other tables (the main reason to use Data Validation is to ensure, that entering information for e.g. unregistered entities like projects, periods, etc., is not allowed except the table where this information must be registered first).

Without registering data entities and using Data Validation when adding them into other tables, there will always be the risk of typos, and when user enters e.g. project name into e.g. project components table, and makes any typo (wrong letter, a space at end, etc.), Excel will see it as a separate project in all calculations, plus you will get error whenever some formula tries to get any additional info for it from projects table.

Another reason having separate tables where various information sets are registered (I often use term Registers for such tables) is, this allows to consolidate full information about any information set in a single table. E.g. when you have in projects table columns for start and end months of project, for name of project manager, for name of client, etc., whenever you add this project into other tables, you always can get this additional info read from projects table, instead entering it manually. And whenever you discover, that some of this info must be corrected, you correct it in projects table in single row, and this change affects all tables and reports in your workbook immediately.