What is a database?
As data visualization software are working databases, let's define what those are.
A database is nothing more than a place which is storing data. Let's dig straight away within it by taking an example. Let's imagine that you would like to list the name of the persons which are in your family. You will then end up with something similar to this
Here we have a database which is composed of a table which has some first names. The first name is the name of your column and Paul, Eric, Susan, Helen, Louise and Franck are nothing more than values. With those values, there is already some calculation that we can do, for example how many first names do I have within this list. Answer is five.
Values, so here Paul, Eric, Susan, Helen, Louise and Franck, have a type. And according to those types you can or cannot make some calculations. Let's take an example, if I add Paul to Franck I wouldn't get anything "PaulFranck" does not have any meaning. Whereas if I was getting their age, I could make an average, for example Paul has 40 and Franck has 41, so the average age of those two men is 40,5 years.
The more data a table has, the more things you can do, there is also links possibilities within tables that make you query others to make even more complex calculations.
The data on which you cannot make calculations on are named dimensions, the other are named metrics.
In order to go further with dimension and metrics, each set of data has a different type. For example here the first names are what we call strings or text, so it means that those fields are expecting to receive any kind of data. Others such as the age here are in fact of a type named integer, which mean that they are recognized as number. Each of those type means that they have properties that you can make data visualization on. For example you cannot calculate a rate on the first names, that wouldn't make sense.