Redundancy is when we store same information in different parts of the database , while this is not possible to remove redundancy completely , it should be avoided whenever we can.
Let’s say you are designing a system for a university. There’s a requirement to store the addresses of people in system as well and you have separated faculty from admin staff and storing them in two tables. Now you are also storing information about student’s addresses as well. Now when you will chose cities for all this different type of people, you may end up adding same city to hundreds of students, teachers and admin staff. Now same city name will be available in different parts of database. This is called redundant data. See below example
You can simple avoid this by separating cities in one table and then just reference the correct. Foreign key to the city table. Now cities in all your database is one table. Congratulations, you have reduced the redundancy from your design.
This a very basic example for redundancy , one may argue that the data is redundant here . But as I said , objective is to minimise it whenever possible .
Hope this will clear your understanding of redundancy in databases.