The Kashmir Shaivism tradition, also called Trika Shaivism, is a non-dualist branch of Shaiva-Shakta Tantra Hinduism that originated in Kashmir after 850 CE. In its place of origin in Kashmir, this tradition is commonly referred to as "Kashmiri Shaivism." It later spread beyond Kashmir, with its great scholar Abhinavagupta calling it "Trika" (meaning "the Trinity"). It particularly flourished in the states of Odisha and Maharashtra. Advaita Vedanta and Kashmir Shaivism (Trika) are both non-dualistic or monistic philosophies, that are very much similar to each other. Being a Vedantavadi I would start with summarizing the basics of the Advaita Vedanta first, and then I'll eventually move on to the Trika Shaivism. The Ultimate Reality in Trika is Paramashiva, and Just like Advaita Vedanta, the nature of Ultimate Reality is Sacchidananda as well. What's unique is that Shiva in Trika system has three additional features- Iccha, Gyana & Kriya Shaktis, and is always endowed with Swarantrya. In other words, Trika philosophy has the same premise as Advaita Vedanta, but gives a bit more detailed explanation of the Ultimate Reality.