What's Meant by Sql Server database primary data file?

Question

When I create database with default size and default path I can't differentiate between its files , When I Go to the path of files of a database I find two files , for example the first one name is School.mdf and the second one name is School_log.ldf , I want to understand what's the difference between School.mdf and School_log.ldf ? , What's meant by Sql server database primary data file and Sql Server database transaction log file?

 

 

thank you very much!