There's lot of definitions for red hat, and is often used without explanation. It's all about context.

Most recently and most frequently it's a reference to Donald Trump's infamous "Make America Great Again" MAGA red hat.

Ironically on the left wing, red is the traditional color of communism. The same red beret worn US airborne troopers were also worn by militant socialists or union organizers. Hugo Chávez, president of Venezuela from 1999 until his death in 2013, was called a red hat.

Pre-World War II, pacifists and union organizers were called red hats. The "Red Hat Acoustic Music Club" celebrates this usage.

Red hat can be used as a reference to the Bloods criminal gang, such as Jay-Z "What's Free" song.

Red-hats / red caps are malevolent spirits found in the lowlands of Scotland. Their red hats would frequently re-dyed in human blood. It's associated with violence and vampirism. It the meaning of AFI's song "Red Hat"

The Linux computer software is called "RedHat" because its creator Marc Ewing wore a red hat.

A Rode Cone(although technically orange)

Someone who typically supports the United States of America. Usually misrepresented people of many ethnicities that support the 45th president.

A supporter of President Donald Trump supporter, because they wear red hats.