Ideographic Description Sequences describe characters from their parts, so 'the reader can then create a mental picture of the ideographs from the description'. IDS are designed to describe rare or new characters not as yet incorporated into Unicode, although they are useful for language learning.
For example, if '亭' was not in Unicode, it could be represented by '⿱⿳亠口冖丁'. This can be compared to describing 'é' as 'e with an acute (upward) accent on it'.
Ideographic Description Sequences (IDS) are formed by 12 'description' characters, ⿰ ⿱ ⿲ ⿳ ⿴ ⿵ ⿶ ⿷ ⿸ ⿹ ⿺ ⿻, and from all of the other pre-existing characters.
IDS are graphical descriptions, and they are subjective, so it is possible to describe a character in more than one way, e.g. '刃' as '⿻刀丶' or '⿹刀丿'. Hence, they do not indicate anything about how the character came to be formed (etymology).
Hanyo Denshi refers to the 'General-purpose electronic information exchange environment maintenance program , character correspondence working committee material' dictionary non-published character material (March 2008 National Institute for Japanese Language and Linguistics)' (auto-translation).
IWDS-1: The IRG Working Document Series 1: List of UCV of Ideograhs
Entity references: Codes beginning with a '&' (e.g. &CDP-88E2, for the sequence for '𥹐') are 'entity references' to characters in other databases than Unicode, such as the Chinese Document Processing (CDP) database (Taiwan). You can look up CDP characters at https://glyphwiki.org , e.g. cdp-8b6c. When possible, these have been removed (see Acknowledgements).
Numbers within circles, e.g. ⑫, indicate a character not shown, but having a certain number of strokes.
References / Further information:
https://www.unicode.org/versions/Unicode9.0.0/ch18.pdf#page=23
http://www.chise.org/ids-find