This is the value that would be obtained in an ideal measurement. An ideal measurement is one that would have no errors at all.
In other words, this is a value that is perfectly accurate.
e.g. If a 1 metre ruler is entirely accurate in length then it will not be even a tiny amount more or less than 1 metre. The problem is that we cannot measure with 100% accuracy! Even if we knew how many atoms there were along the length of the ruler, we can never expect to know their separation and size exactly either.
In other words, the 'true value' is hypothetical – no such thing can actually be measured.