he was by profession a jurist, and participated extensively in the legal discussion of Weimar Republic (his work before Weimar wasn't that notable or extensive yet)
as you must be aware, the Republic was in a great legitimation crisis, with major problems both outside and inside its borders and the new fledgling parliamentary-republican government trying to maintain control
Schmitt thought that the spirit of the Weimar constitution laid in its presidential, ie. executive power provisions, and not in the parliamentary system, which he deemed inadequate to produce real decision (de facto correct). Schmitt made an interesting distinction here between democracy and liberalism, two very different things, and parliamentarism belonging to the intellectual world of liberalism, not democracy
His intellectual enemies at the time were mostly other jurists, more specifically the legal positivist tradition exemplified by Hans Kelsen. Simplifying here, Kelsen argued that every legal norm is based on another legal norm. This leads to a self-fulfilling circle with no explanation to how the constitution first comes into power or why it has to be followed. Kelsen thought that legal theory had to be purified of extra-legal matter, such as psychology and the political.
Schmitt's notable contribution here was to point out that since norms cannot choose to apply themselves, we have to consider extra-legal matters, ergo the person or other agent who decides whether or not a law is in force or not. The height of this decision, for Schmitt, is the capability to decide on the exception, ergo an emergency situation which the political community must confront, and in which the whole constitution might be withheld. The person who is capable of making this decision is the sovereign in any given situation. So: Schmitt's major contribution is in the discussion surrounding emergency powers, which in the last years have been very relevant because of COVID-19 or e.g. South Korea