It is just a code that identifies a particular Fare -- please refer to the Fare Rules for all the details.
To be a bit more precise: the 1st letter generally refers to the fare bucket which the fare books into. The rest of the code normally summarises the fare in some way - but each airline uses their own system for this. If you regularly look at the fare basis codes for a specific airline, you may be able to identify the patterns.
For example, looking at AF (and KL)
(incomplete extract):
[KVS Availability Tool 6.9.9.R3/Diamond - ATP: Fares/FC/US]
Code:
CDG Paris Charles De Gaulle FR [LFPG]
NYC New York Metro NY US = JFK LGA
R/T
Carrier From To Fare Cur Total Q YQYR Fare Basis
--------- ------ ---- -------- ---- -------- --- ---- -------------
AF PAR NYC 645 USD 996 351 LLSBFR
AF PAR NYC 645 USD 996 351 LLSRFR
AF PAR NYC 697 USD 1048 351 LKSBFR
AF PAR NYC 697 USD 1048 351 LKSRFR
AF PAR NYC 812 USD 1163 351 HLSRFR
AF PAR NYC 863 USD 1214 351 HKSRFR
AF PAR NYC 1068 USD 1419 351 KLSFFR
AF PAR NYC 1119 USD 1470 351 KKSFFR
AF PAR NYC 1145 USD 1496 351 LHSBFR
AF PAR NYC 1145 USD 1496 351 LHSRFR
AF PAR NYC 1311 USD 1662 351 HHSRFR
AF PAR NYC 1562 USD 1913 351 ULSFFR
AF PAR NYC 1567 USD 1918 351 KHSFFR
AF PAR NYC 1644 USD 1995 351 UKSFFR
AF PAR NYC 1772 USD 2123 351 UHSFFR
AF PAR NYC 1818 USD 2169 351 SLLSFR
AF PAR NYC 1900 USD 2251 351 SKLSFR
AF PAR NYC 2067 USD 2418 351 SHLSFR
AF PAR NYC 3219 USD 3570 351 SFFFR
we can deduce that:
1st letter = fare bucket
2nd letter = seasonality (L = low season, K = shoulder, H = high; not present for fares with no seasonality)
3rd, 4th letters = degree of flexibilty (SF = semi-flex, FF = fully flex; not sure about SB, SR or LS)
5th, 6th letters = country of origin (FR = France in this case)
For some airlines, the maximum validity will be included in the fare basis code (eg: 3M = 3 months, 1Y = 1 year, etc)
For DL, I'd guess that the number in the middle is the advance purchase requirement (eg: 21 = must be purchased 21 days in advance). However, I don't have sufficient experience to be able interpret the rest of the DL fare codes yet.
As has been said, full information is contained in the fare rules, in any case.