What is US' Patriot missile, supplies of which are globally depleted?

Patriot missiles are surface-to-air interceptors used in US' MIM‑104 Patriot air defence system and are currently facing a global shortage. The long-range system built by Raytheon Technologies primarily uses two missile families, PAC-2 and PAC-3. Depending on the missile used, the system can intercept aircraft, tactical ballistic missiles and cruise missiles. One system can cost over $1 billion.