Its a race

Bicycle racing can be confusing to some. But the Tour is pretty simple. You are racing to get to Paris in the least amount of time. The rider who gets to the finish line fastest wins the race.
The Tour is made up of several smaller races - 21 this year - and the times of these races are added to get the overall time. The smaller races are called stages.
Each stage has its own characteristic.

10 flat stages,
5 mountain stages,
4 medium mountain stages,
2 individual time-trial stages.

The riders work as a team with a designated leader. Usually the team leader is the one they want to get over the finish line first. But that is not always the case and the team will work for the rider who is doing best.

On the flat stages the team works to get the group or "peleton" together and often ends with a mass sprint at the finish line.
On the mountain stages the team works as a group to pull their leader over the summits so they are "rested" to race for the finish lines.
The individual time-trial is man against the clock.

This year the Tour organizers has made a few changes that should help keep the race simple.
The best is the elimination of time bonuses for stage winners. In the past few years if you finished in the top 10 or so per stage you would get an extra second or two taken way from your time. This year's race has no time bonuses so it will be the true race time that wins.