This is the 26th most frequent Hebrew word.
הזה
"הזה" in Hebrew means "this" (masculine singular).
'הזה' (ha-zeh) in this example provides emphasis to the noun, 'הרעיון הפשוט' (the simple idea), illustrating that it refers to a specific, previously known concept.
זה הרעיון הפשוט הזה שהתעלמו ממנו.
It's this simple idea that was ignored.
In this sentence, 'הזה' (ha-zeh) functions as a demonstrative pronoun, specifying a particular noun, 'הספר' (the book).
הספר הזה מרתק.
This book is fascinating.
Here, 'הזה' (ha-zeh) again acts as a demonstrative pronoun, referring to the specific noun, 'הרחוב' (the street), which it follows.