Sniffing is a process of intercepting data packages on a given network. Whatever interaction that you take on a device that requires a network it will send data trough it. This data can be intercepted and analyzed by a sniffer.
Sniffers can be both software and hardware and will see as much as the network configuration. For example, if you connect to open wifi with a sniffer, you can intercept all data packages of people connected to that access point on the fly.
Sniffer tools are used by network administrators to troubleshoot, yet a bad actor can also exploit them.
The exploit consists of intercepting and reading those packages. Fortunately, most new services encrypt their traffic so the result of such interception would be gibberish. If the protocols used to send data are not secure with encryption, then all data will be readable by the attacker.
It is one of the reasons that it is of good practice to not trust any open Wifi.