Recoll is a full text search application. Full text search applications let you find your data by content rather than by external attributes (like a file name). More specifically, they will let you specify words (terms) that should or should not appear in the text you are looking for, and return a list of matching documents, ordered so that the most relevant documents will appear first.
You do not need to remember in what file or email message you stored a given piece of information. You just ask for related terms, and the tool will return a list of documents where those terms are prominent.
This mode of operation has been made very familiar by www search engines.
The notion of relevance is a difficult one, as only you, the user, actually know which documents are relevant to your search, and the application can only try a guess. The quality of this guess is probably the most important element for a search application.
In many cases, you are looking for all the forms of a word, not for a specific form or spelling. These different forms may include plurals, different tenses for a verb, or terms derived from the same root or stem (exemple: floor, floors, floored, floorings...). Recoll will by default expand queries to all such related terms (words that reduce to the same stem). This expansion can be disabled at search time.
Stemming, by itself, does not provide for misspellings or phonetic searches. Recoll currently does not support these.