The McMahon Line is like an imaginary line that goes through the mountains between India and China. It is named after a man called Sir Henry McMahon, who drew the line on a map in 1914.
Now, India and China were not very happy with each other at that time (they were actually quite angry with each other!). So the idea of the McMahon Line was to show where India's land ended and China's land began. McMahon put the line there because he thought that was where the people living on either side of the mountains had always thought their borders were.
But, China didn't agree with the McMahon Line. They thought that the line was drawn without properly considering their point of view. This led to a lot of protests and disagreements between the two countries, which still continue to this day.
So, even though the McMahon Line is just an imaginary line drawn on a paper, it is a very important line that helps separate India and China.