I have been working on an RF project and wanted to know the RF noise floor present in the lab. I connected the Lora Antenna to the spectrum analyzer as shown in the image below and set the Marker at 915MHz frequency point. It shows a value of -89.87dBm. I want to know what does this -89.87dBm mean? Does it mean the ambient noise level present in the lab at this frequency?
That's the intensity of the signal from the antenna in that instant, in the 300kHz BW around 915MHz and with you antenna placed exactly in that position. It's tricky and that's why they invented shielded rooms.
How to proceed from that depends on the kind of testing you need to do (power output, harmonics, modulation bandwidth or whatever). As a first approximation however, yes, that's your noise floor. Remember that the scope is logarithmic so you'll need a comparatively bigger signal to raise the indication (-90dBm is a typical receiver sensitivity)
You are measuring the noise floor in the frequency band received by the antenna and zoomed in by the instruments in the range 868 - 915 MHz.
If you want to measure the noise floor in the range 9 KHz - 1 GHz you need a broad band antenna or a set of antennas that cover that full range.
Noise floor is important, and it's a figure of merit, of semi-anechoic or fully anechoic chambers.