Starting in 2008, Hot Wheels began using base codes. These base codes can be interpreted as follows.

Letters = Year of Manufacture

A 2008
B 2009
C 2010
D 2011
E 2012
F 2013
G 2014
H 2015
J 2016
K 2017
L 2018

Numbers = Week of the year

1-52 Week of the Year

Example #1: B12, B indicates the car was manufactured in 2009. 12 being the 12th week of the year puts the production date approximately the end of March.

Example #2: C47, with C indicating that the car was manufactured in 2010. The number "47" indicates the car being manufactured in the 47th week of 2010, which puts this car's production date around the end of November.
Other than helping to search for new variations, these base codes have little meaning among collectors.