The phrase "glassy conditions" refers to the state of a surface, usually water, being smooth and shiny like glass. It means there are no visible waves or disturbances, creating a calm and serene environment. Full definition
«I think the ideal surf is when the swell is southwest and the tide is low, going into high tide, with glassy conditions in the morning. (ticotimes.net)
We've had glassy conditions in the morning and at sunset. (coconutharrys.com)
With glassy conditions on Kong's and Money Trees with 4 surfers in the water. (baliwaves.com)