dB is decibel, a measure of 'loudness', or more strictly, power. The figure of 35dB represents the amount of hearing loss, effectively a raised hearing threshold at a specific frequency.
Hz is Hertz, or frequency in cycles per second. Hearing is usually measured at specific frequencies of pure tones, with different limits for allowable loss tailored to match human hearing sensitivity.
To help you, Middle C on a piano is aprox. 250Hz and is considered a 'low to mid' note, 1000Hz or 1 kHz is 2 octaves above that, and is considered a 'mid' frequency. 4kHz or higher is 2 more octaves higher in pitch, and is considered 'high frequency', the region where 'Ts' and 'Ss' consonants are formed. These are the frequencies which are most usually lost first in any hearing loss. This causes loss of detail in the sounds or speech which can be understood, leading to a muffled or unclear sound.
A typical hearing test would cover discrete frequencies across the audible range from 125 Hz to 8000Hz.