"Demonstrate" means "to show something."
Example #1: These numbers clearly demonstrate the size of the economic problem facing the country.
Example #2: The company demonstrated an app for surgeons that showed X-rays on the screen.
"Demonstrate" also can mean "to march or gather somewhere to show your opposition to something or your support for something."
Example #3: Thousands of people demonstrated outside the parliament building.
Example #4: 30,000 angry farmers demonstrated against possible cuts in subsidies.
Video by For Your English: https://www.youtube.com/channel/UCMarCnGQD3s5f9oc5Icn4sw