Trump wants to build a wall. He's currently trying to get congress to fund it by negotiating with Democrats. This isn't going well. Trump has threatened to declare a national emergency at the border in order to sidestep Congress and the Democrats.
The "emergency" is that Mexicans are bringing crime, drugs, and terrorists over the border. Claims that range from dubious to flat out wrong. The president, after declaring a state of emergency, is basically given temporary power to suspend certain laws, use the military in some ways without congressional approval, or drafting retired servicemen, etc. This is so in the case of a terrorist attack, disease outbreak, surprise war declaration, the president can take actions to deal with the situation without waiting for congress's approval.
Previous examples are events like 9/11:
or the swine flu outbreak:
What he wants to invoke is invoke a national emergency so he can use the military to construct the wall:
https://www.law.cornell.edu/uscode/text/10/2808
The thing is, no such state of emergency exists. Everything he offers as justification for it is just false. What he's doing is he's trying to exercise emergency powers illegitimately to sidestep congress and fulfill campaign promises to appease his base. This directly violates the constitution and subverts the balance of power. It's a direct attack on the constitution and congressional authority.
You cannot abuse the spirit of the constitution nor its letter to fulfill campaign promises. Personal liberty aside, even the threat to do this is fascist. He doesn't give a damn about the constitution and wants it gone so he can have more power.