The team that wins the toss can defer their options to the second. What this does is allows the other team to choice how the game will begin. The choices are kick or receive, or which side they'd like to defend.
An example:
Team A wins the toss and defers.
Team B chooses to receive.
Team A then has to decide which side of the field they'd like to defend.
Second half options are now with Team B:
Team B chooses to receive
Team A then has to decide which side of the field they'd like to defend.
In windy games I have seen this scenario
Team A wins the toss and defers
Team B chooses to defend the south goal
Team A chooses to receive
Second half:
Team A elects to receive
Team B elects to defend the south goal.
What's important to note in these examples is that no matter what choices are made for the first half, the second half options must be chosen again. There was always a myth that fans believed that the team that kicks in the first half was guaranteed to receive in the second half. That is typically what happens but it is the teams that decide who kicks based upon who has the options. If Team A wins the toss and chooses to kick for whatever reason, Team B gets the second half options and they will likely choose to receive, which it is obvious that they are receiving both halves.