Neti-neti is a Sanskrit expression that translates to “neither this, nor that” or “not this, not this.” This expression is used in Hinduism, mainly in Jnana yoga and in Advaita Vedanta (non-dualistic spiritual practice). Neti-neti is a form of analytical meditation that helps the individual understand the nature of brahman (absolute reality) by first understanding what is not brahman.
A “neti-neti search” is one of the key elements of Jnana yoga practice. It is an exercise in negating rationalizations and other distractions from the non-conceptual meditative awareness of reality.
Yogapedia: In Jnana yoga and Advaita Vedanta, “neti-neti” may be a chant or mantra. It is an 8,000-year-old practice that takes the focus away from all disturbances, so the practitioner may find the stillness in every movement and the formless in every form. Neti-neti meditation helps to identify all things of the world which are not the atman (the real), thus negating the anatman (the unreal).
When it’s adapted for the modern world, including neti-neti meditation in one’s yoga practice, neti-neti helps the practitioner realize that they are not actually the things that they normally identify themselves with (personalities, thoughts, feelings or jobs), nor are they merely their roles (parents, partners, friends or siblings) in life. In this way, neti-neti can also be interpreted as “beyond this, beyond that.”
Source: https://www.yogapedia.com/definition/5325/neti-neti