Mysql is relational db with high performance. In SQL we need to define the relations of our data tables. This requires good design, but makes it hard to change the data structure.
Also need to define when adding coloumns. Removing coloumns is even riskier.
In nosql, which stands for Not Only SQL, the data can be loose. Like in json, fields can be added or removed, not restricted by other members in the collection.
For example if we have studends table with the following fields - name, id, phone. In mongo we can add a student with field mobilephone, without the need to modify the existing students. In mysql we need to add a coloumn and define the default value to set for the existing students.
While mongo is more flexible, this is its advantage and disadvantage.
Restrictions makes developement slower, but forces you to think about your data structure and design.
Quick growing agile projects may prefer nosql, while heavy projects, which require high performance probably choose mysql or other relational db.