Here's a simple example for addition which can be extended to other operations as well.
Suppose there $X$ number of children in a classroom and everyone has a score ranging from 0 to 100. We want to calculate the average of the scores of all students without the teacher or the other students knowing what their classmates have scored.
Here's how we can achieve this.
Step 1 :
Make them each right down the score that they have got on a piece of paper and make sure no one is colluding. This only works if everyone does it honestly.
Tell each student to select $X$ numbers and write them down preferably on small bits of paper since there are $X$ number of students such that these numbers will add up to the score they want to keep secret. The range can be from $-∞$ to $+∞$.
Step 3 :
Tell each student to distribute $ X-1$ bits to every other student so that each students ends up with $X$ bits in the end.
Step 4 : Now that each student has $X$ bits of paper , tell them to add up all numbers they have in front of them and write the value they got on the board.
Step 5: Add all these numbers and you will get average of the scores of each student.
You could calculate the average without anyone knowing what their score was.
You can cross verify by taking the average of the actual scores.