The sit and reach test is part of a general health assessment battery of tests. The basic method is to sit on the floor, legs flat, with feet against a box or other vertical stop. A measuring stick or device is used to see how far a person can reach towards (or past) their toes. It's used as a measure of general health when included in a full assessment. Poor results are an indicator that the person may be predisposed towards lower back pain and/or injury unless corrected.
The main limiters for this test are going to be your hamstrings, your lower back (erector spinae), and possibly to some extent the gluteal muscles.
While somewhat self serving, this answer covers all (And then some) of the stretches that will help with this (There are many many other stretches, these are the main ones. Every stretch/muscle group will have multiple variants). The particular ones that you want to do for this specific test would be 1, 2, 4, 7 and 8.