Full-text fields (or just fields for brevity) are the textual document contents that get indexed by Manticore, and can be (quickly) searched for keywords.
Fields are named, and you can limit your searches to a single field (eg. search through “title” only) or a subset of fields (eg. to “title” and “abstract” only). Manticore index format generally supports up to 256 fields.
Note that the original contents of the fields are not stored in the Manticore index. The text that you send to Manticore gets processed, and a full-text index (a special data structure that enables quick searches for a keyword) gets built from that text. But the original text contents are then simply discarded. Manticore assumes that you store those contents elsewhere anyway.
Moreover, it is impossible to fully reconstruct the original text, because the specific whitespace, capitalization, punctuation, etc will all be lost during indexing. It is theoretically possible to partially reconstruct a given document from the Manticore full-text index, but that would be a slow process (especially if the CRC dictionary is used, which does not even store the original keywords and works with their hashes instead).