When testing the preparation of the sub base preparation for the construction of a new concrete road pavement. Why is the CBR determined and what does it mean?

CBR stands for California Bearing Ratio, a standard test to determine the quality of subgrade or subbases. It is an ASTM standard (ASTM D1883 for soil, or ASTM D3668 for soil-lime mixtures).

In general, the quality of your subgrade or subbase determines the thickness of the materials above it (subbases, asphalt or concrete slabs).

According to The Asphalt Institute for design of flexible pavement, a good to excellent subgrade can retain substantial amount of loading capacity, typically has CBR=17, R-value = 43 or resilient modulus around 170 MPa.
Medium subgrade soils retain a moderate degree of firmness under adverse moisture condition, with CBR=8, R-value=20 or Resilient Modulus = 80 MPa.
Poor subgrade consisting of soils which becomes quite soft and plastic when wet, CBR=3, R value=6, Res. Modulus= 30 MPa.

For subbases consisting of good quality, crushed granular (Gran A), it should have a CBR 100.  Granular B should typically has lower CBR, in the order or 60-80.  Gran A has max 7% fines (-#200), non plastic, etc.

Hope that this helps.

In addition to Georam's comments, basically someone designed the pavement structure based on furnished vehicular traffic information, and  design CBR values  for the subgrade, and other layers.  It is important that this design CBR is achieved (or exceeded) in the field, otherwise, the constructed pavement will not perform as intended. The design CBR value can be confirmed by performing field CBR tests.  Once the contractor has his site preparation/compaction methods down straight, periodic testing can be performed as a check, if warranted.