It’s the number of recordables you have per 100 employees. Really easy to calculate too: https://www.ecesis.net/TRIR-Calculator.aspx - you can compare your values to the BLS data to see how you compare to national averages https://www.bls.gov/iif/oshwc/osh/os/summ1_00_2019.htm#soii_n17_as_t1.f.1