The trade deadline is something that comes and goes every season. It is the last day for trades to be agreed to during a season. Now, this does not mean that roster changes can no longer happen. This is important to remember. Adding and dropping players is still allowed after the trade deadline. However, if there is a good player on another team in the league, the trade deadline is your last opportunity to try and acquire those players.
The trade deadline can be different from league to league. The website will offer a handful of dates to choose from that are all around the same time. In fantasy football leagues, it usually takes a couple days for the trade to officially process. If a trade is made on the same day as the trade deadline, the trade is still considered valid as it was made before the deadline passed, even though the two teams involved in the trade will not officially receive their newly acquired players until after the deadline.
There are multiple ways to approach the deadline. If your team is good, you will want to make trade moves well before the trade deadline, as it gives the teams you are trying to trade with more leverage. If you get ahead of the pack and try to make bigger trades earlier in the season, the team you are trading with does not have as much leverage.
Conversely, if you are a bad team that has a few good players, you should wait to make a trade, as you can attempt to take advantage of teams' desperation to acquire good players and, as a result, get a good return. Do not make a trade the moment things turn bad for your team.