Near-IR spectroscopy measures the broad overtone and combination bands of some of the fundamental vibrations (only the higher frequency modes)
Indeed that sentence should be taken as merely anticipating what the book will treat in following chapters.

Nevertheless, to satisfy your impelling curiosity the statement trace a combination of facts that leads to the actual spectrum of a given molecule.

Overtones IR absorption bands are those resulting from breaking of selection rules. Instead of having just transitions between adjacent vibrational levels, basically the 1&lt;-- 0 one, transitions with a delta nu of 2 are observed as well. In other words the vibrations aren't those of an harmonic oscillator but those described by a Morse's potential.

Combination modes arise from the combination of normal modes, ie they happen when different vibrations are simultaneously excited. Normal modes which are unseen can contribute to these combination modes, as the coupling alter the symmetry. 

High frequency: we are speaking about NIR, which is the high frequency range for molecular motions, and it is where vibrational overtones typically occur. The lower frequency part of the spectrum is dominated by the fundamental modes, and except for its fingerprints part and fine details, it has a strong bond related character rather than a molecular one. 

If this does not help but instead brings out new terms or doubts, I would suggest that you treat the introductory part of the book for what it is, namely an introduction.

Alternatively you can refer to this link which provides a condensed explanation and includes schemes, further links, and definitions:

http://bit.ly/31Mc6lT