What does the points under every question in a contest means?

Hello , i am new to codechef . While solving the questions in contests i have seen ,there are some points written for every question.
can anyone tell me what these points mean, how they are linked to rating of a user.
waiting for a humble reply.

In contests by solving questions,you get points. Getting more points means better rank in contest which means your rating may increase.
This is a very simple understanding , read the blogs and posts regarding these topics to know more.
In short explore.

for joining DIV1 during a contest it shows ,my rating should be above 1800.
Does it mean that i have to collect more that 1800 points by solving questions in DIV2 contest.

Exactly. Those points are also called rating. How many points you gain after each contest (or the rating increase) depends on your performance relative to others. If you solve more questions in some contest (time of submission is also important in short contests) than the people with similar rating , your rating will increase.