CBRS stands for Citizens Broadband Radio Service. It is a block of spectrum in 3.5 Ghz with the range of 150 Mhz (3.5 (3550 MHz to 3700 MHz) for shared wireless access. This used to be used by US Navy and other DoD members, but freed up for unlicensed wireless application like LTE (Refer to What is CBRS? - LTE in 3.5 GHz Shared Spectrum and What it Means for IoT ) for the details.
As mentioned above, this is unlicensed spectrum meaning that you don't have to pay the cost for the spectrum. You can think of this as similar to ISM band, but how you can get a spectrum in the block. In case of ISM, you can just turn on your device working in the spectrum without any special procedure as long as the device meets the regional transmission requirement (e.g, FCC). However, you need to go through a special procedure to get a section of the frequency band as described in What is CBRS? - LTE in 3.5 GHz Shared Spectrum and What it Means for IoT
Now this band is open for 5G/NR as well (Ref )
 3.5 GHz Band Overview (FCC)