The Department of Homeland Security has decided to continue limiting travel from Canada into the United States at land ports of entry along the border. Travel is restricted to "essential travel" only, which includes activities like returning U.S. citizens, medical purposes, and lawful trade. Non-essential travel, such as tourism, is not allowed under this rule. These restrictions are temporary and will be in place until March 21, 2021, but can be adjusted as needed.
Simple Explanation
The people in charge of keeping the U.S. safe decided that only important trips from Canada to the U.S. are allowed right now, like going to work, visiting a doctor, or if you're coming home. Fun trips, like going on vacation, will have to wait until at least March 21, 2021.