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noise floor

ok i know this is very basic question and i feel like an idiot for asking so dont chastise me any further.
i think i understand what noise floor is but could someone further explain this to me (yes i have searched it but to no avail) and how i can measure it in my studio.
is it basically just recording silence with all equipment on?
jesus i feel like an idiot.

noise floor is changing during recording

hello.
i was recording my synthesizer SY 85 into Apogee trak2 through coaxial digital cable using Sony Sound forge apps.
even though i didn't play SY 85, the noise floor level is changing
why did it happen?
the noise level was -69 dbfs, but suddenly it was changed to -79dbfs. then again it was -79dbfs