There’s a short video on the No Kings site that explains what the protest is for.

We’re protesting Trump and his use of authoritarian power against the will of the people. Such as:

• The kidnapping families off the street to deport them without due process (illegal)
• Illegally sending the National Guard and Marines to LA to stop peaceful protests against the will of the people and Governor
• Taking away Medicaid from hundreds of thousands of people, many of whom may die without it
• Making ICE into what is effectively Brown Shirts
• Persecuting his political enemies, journalists, and lawyers, who dare stand against him