First of all we shall discuss what the mean of something is. As the name suggests, mean is the real average of all the things that have been recorded in a survey. For example, a class has given a quiz to their teacher, the marks of this quiz are given back and the teacher needs to note down the average of the class in that particular quiz. Now what the teacher will do is that he will note down all the marks of the separate students on one paper and will add them together. After adding them together, the teacher will calculate the total sum of all these numbers. He will then divide this sum of numbers with the total number of students that are present in the class that have taken the quiz. The result will be the mean.
Median is another way. For the same example, the teacher will write the numbers of the students in a decreasing order on a paper and then the middle value of all these numbers will be the median.
The final thing is the mode. Mode is the most repeated number of the entire list and this is also a way to calculate the average. Note that the method will generate different results.