Fidelity refers to faithful recording of the information known to the data entry users.
Consider cases where user-input is in itself valid and reasonable and might be expected to succeed but the database refuses to record the update because it conflicts with other information already in the database, perhaps entered by some other user.
The DBMS determines there's a conflict because of a violation of an agreed unassailable truth about what's actually possible about the world but is in effect guessing that the proposed update is incorrect, as though the information already in the database is unassailable.
For example, in a births relation variable for a genealogy database one user thinks Mary Smith was born on Jan 3 1900 and another user thinks Mary Smith was born on Jul 3 1900. A key constraint doesn't allow the disagreement to be recorded. Of course we know Mary Smith cannot have been born a second time 6 months later.
Missing information inevitably happens in reality. Consider that the address of an employee is unknown. If the relvar predicates are statements about what is supposed to be the case rather than only what is known to be the case then someone is either forced to enter a fictitious value, or not enter the record at all. That the opposite of fidelity / integrity. Paradoxically constraints can mean a lack of integrity.
Predicates of the form "it is known..." allow for entering information as it becomes known - i.e. piecemeal, which is very practical. It is a good idea for a DBMS to manage information as it is entered.