The phrase "unstable surface" refers to a ground or platform that is not steady or secure, making it easily prone to movement or shaking. Full definition
Many strength coaches recommend against training on unstable surfaces when using free weights for improving sports performance. (strengthandconditioningresearch.com)
This could mean that the balance aspect of unstable surface training could lead to strength gains irrespective of the loading used. (strengthandconditioningresearch.com)