You really need to read the GROMACS manual on how the dihedrals field is formatted. Very briefly:

Numbers 1-4: the atom numbers of the four atoms defining the dihedral (e.g. atoms 48, 43, 38, 42).

Number 5: a number indicating which dihedral function is used. GROMACS supports a number of different functions for each interaction type, each indicated by a number. This value also defines what other information follows, and what it means. For example, a value of "9" is the multiple dihedral scheme used by force fields such as CHARMM. In your case, Function "3" is the Ryckaert-Bellemans function that GROMACS uses to model dihedrals in OPLS.

The rest of the numbers are the Ryckaert-Bellemans parameters used in the dihedrals, c0, c1, c2, c3, c4 and c5. These can be derived from the Fourier type dihedrals of original OPLS through the equations 32-33-34 in the following page of the manual:

http://manual.gromacs.org/documentation/current/reference-manual/functions/bonded-interactions.html#proper-dihedrals-ryckaert-bellemans-function

Edit: Also, read the following page of the manual. It basically describes the top/itp format. In table 14, you can find all bonded interaction forms, as well as a short description of their fields:

http://manual.gromacs.org/documentation/current/reference-manual/topologies/topology-file-formats.html#tab-topfile2

www.few.vu.nl/~feenstra/articles/Gromacs2.0Manual.pdf