Well, that's not the only way to look at the phenomenon, and it's not necessary
to believe that there are gaps like black holes left in sentences or anything like that.
But there's a point it makes.
There are two great families of pronouns, both descending from Proto-Indo-European roots:
Demonstrative pronouns and their allies, descended from the PIE root *to-
In English these start with TH: there, that, thither, thence, then, this, thus, though, the, they, etc.
Relative and interrogative pronouns and their allies, descended from the PIE root *kʷo-
In English these start with WH: where, what, whither, whence, when, how, who, why, whether, etc.
So if that is a relative pronoun, it's an odd one.
The thing is that that has been used for over 600 years to introduce tensed subordinate clauses.
And it still is used that way for two out of the three kinds of subordinate clauses in Modern English.
Tensed Noun clauses and Adjective clauses require or allow that, under varying rules:
- Mary believes [that [the Earth is an oblate spheroid]]. [Complement (Noun) clause]
- The man [that [Mary talked to]] didn't believe her. [Relative (Adjective) clause]
Tensed Adverb clauses don't allow it any more, though they used to in Chaucer's day:
- When he gets here, be sure to offer him a drink. (grammatical without that)
- *When that he gets here, be sure to offer him a drink. (ungrammatical with that)
But compare the first line of Chaucer:
- Whan that aprill with his shoures soote the droghte of march hath perced to the roote
'When (*that) April with its showers sweet the drought of March has pierced to the root'
Now the kind of word that introduces a clause and doesn't have meaning as such
is a Complementizer. Clearly this is the right term for the that in that the Earth is an oblate spheroid.
And it's obvious that it's the term for the way Chaucer used it, too.
So the question is, what is it when it heads a relative clause?
If that is a relative pronoun, then there are a lot of questions that need answering:
Why is that so different from the other relative pronouns?
Why is that optional in restrictive relatives but forbidden in non-restrictive ones?
Why can that be used in any kind of clause, where WH relatives are specific to person, time, etc?
Why is it being used as a relative pronoun when it's already used as a complementizer?
On the other hand,
If that is a complementizer in relative clauses, then these questions evaporate.
What appears then is a situation similar to complement clauses.
That may be deleted in complements, just as it can be in relative clauses, and under the same conditions
-- whenever it's not needed to mark the subordinate clause as a subordinate clause, it's optional.
This also solves all coreference problems, because complementizers are not referential.
Therefore if there is any coreference in the relative clause, it is coreference of the sort called "understood" (also, "deleted", "a gap", "zero", "Pro" (of two varieties), or some other term, depending usually on what year your syntax instructor got their PhD).
That's what they really mean, I think. There's quite a lot more one could say on the topic,
but I forbear.