This is the 6105th most frequent Catalan word.
llacuna
"Llacuna" refers to a gap, omission, or a small lake/pond.
In this sentence, 'llacuna' refers to a missing or incomplete part within the content of the book.
La llacuna al llibre està en el capítol tres.
The gap in the book is in chapter three.
Here 'llacuna' is used to denote a natural body of water typically small in size, like a pond.
La llacuna d'aigua al bosc és tranquil·la i pura.
The water pond in the forest is calm and pure.
'Llacuna' in this usage points to an absence or deficiency in understanding or expertise.