What is the Insurrection Act that Trump is threatening to invoke?

Photo in the story [0], taken late today, is of Trump with the highest-ranking military officer in the US Armed Forces (on the right, [1]) - and the US Attorney General (on the left, [2]).

Trump intentionally left the White House to force his way through the protesters.

It appears likely it is only matter of time before Trump deploys the military.

[0] https://media.npr.org/assets/img/2020/06/01/gettyimages-1216...

[1] https://en.wikipedia.org/wiki/Mark_A._Milley

[2] https://en.wikipedia.org/wiki/William_Barr