In addition to the page that La Texican gave you, this table gives you a good idea of what levels correspond to which grade levels: http://teacher.scholastic.com/products/guidedreading/leveling_chart.htm

Not sure how your child was assessed, but generally grade level skill is a combination of decoding/word reading ability, accuracy & rate (fluency), and comprehension. QRI (qualitative reading inventory), DAR (diagnostic assessment of reading), DRA (diagnostic reading assessment), all attempt to quantify a child's reading level by looking at each component of reading.

Generally speaking, a parent can "test" whether a book is a good fit from a decoding/word reading standpoint, by having the child read a page of text. If they read 95% of the words correctly (the first time, no self corrections), the book is at their independent reading level. 90% would be their instructional level, and anything below would be at the frustration level.

If you want to check if the child's fluency (rate and accuracy) is at or above grade level, have them read a passage of text at their independent level and count how many correct words they read in one minute. You can compare their rate to this chart: http://www.readnaturally.com/pdf/oralreadingfluency.pdf