What is SENSEX and Nifty? How does it go up and down?

In very simple terms

consider it this way:

NSE/ BSE are schools (we call exchanges)

Stocks are the students (companies are registered called scrip or stock)

now these students are divided into categories

so, the best top 50 students are categorized and kept in a class named Nifty

second best 50 students are categorized as Nifty Next 50, then next best as Nifty midcap 50, small cap, Liquid stocks, FNO securities etc...

so, value Nifty is average market capitalization of those 50 companies like average marks of those 50 students in class Nifty increase or decrease in level depends on change in their price

market capitalization: current market price * total number of shares