The 'playoffs' is the fantasy football postseason that includes the final set of teams playing for the league championship.
There are many ways the playoffs in fantasy football can be structured. If the league is sorted into divisions (small groups of multiple teams within the league), usually the top team in the division will advance. There is also often one or two 'wildcard' playoff spots available to the best teams that did not finish at the top of their division. If there are no divisions, only a limited amount of the top teams will advance to the playoffs.
The playoffs are usually structured into a bracket. Based on the number of playoff teams, the best records in the playoffs could be awarded a 'bye week' during the first week of the playoffs. The playoffs often only consist of the top six or eight teams in the league, with two receiving 'bye weeks'.
Each week, teams in the playoffs will play an 'elimination game' wherein the team that loses will be eliminated from the playoffs. The winner of the final playoff game is the champion of the league.
In case there are any ties in overall record between multiple teams going into the playoffs, a tie is usually broken by whomever has the most overall fantasy points throughout the season, whoever won the head to head matchup, or which team has the most victories against common opponents between the teams.
It is important for the league commissioner to schedule the playoffs so that the championship falls on the 16th week of the season rather than the 17th, because many teams rest their starters going into the actual NFL playoffs which would be detrimental to high caliber fantasy rosters.
Firstly, know where your players will be playing. It will be around December and January by the time the fantasy football season winds down, and teams playing in the north will be playing in cold and snowy conditions. This is also where knowing your players strengths and weaknesses comes in handy as some players may thrive in inclement conditions.
Note that during rain and snow, the running game is much more favored than the passing game and in indoor stadiums and warm conditions, passing and kicking are favored over the running game.
Finally, do not overthink any of your decisions. If you were solid enough to make it to the playoffs, you're obviously doing a good job.