What do 5-sigma, r, and lowercase-delta mean in the physical descriptions of major telescopes?
The quote appears to be from the Ivezić et al. 2019 paper about LSST.
In this context, σ quantifies the noise level in the images, and r is a photometric passband of ~620±70 nm wavelength.
In single r-band images, they expect stars of AB magnitude 24.5 to stand out 5σ above the background level.
δ refers to declination; they plan to cover the sky from the south celestial pole to 34.5° north of the equator.