A freeroll league is a league in fantasy football where there are no buy-ins, but there are still cash prizes to win. These leagues are risk-free, as you can not lose any money. However, since there is still a payout, there is some incentive to win.
Freerolls are offered on fantasy betting sites such as DraftKings and FanDuel. If you have the chance to enter a freeroll, you should take it! You can't lose money but you can win big, so it's a great opportunity.
Most freerolls are daily fantasy games. On DraftKings, freerolls take place on Sundays, and exclude Thursday games. Remember that daily fantasy gametypes carry a different strategy than year long leagues.
Some freeroll leagues can use the standard fantasy football league format. These are rare however, as someone has to provide the money that is used for cash prizes. If this is the type of league you are in, consider the normal fantasy football league strategies.