The "LF" means a Littelfuse Part. The casing is SMC, meaning it is likely a Transient Voltage Suppressor (TVS). They make an SMC part with the number SMCJ120CA, which seems like a safe bet as to what this is. The 120C is a bidirectional 120V part.
The job of this part is to limit the voltage on the line it is connected to. In normal operation, it does not conduct, but above a certain threshold, it turns on, effectively clamping the voltage between the terminals like a diode. In this case, it is meant to connect to a line that is nominally between +/- 120V, starts conducting at +/- 133, and will limit its voltage to +/- 193V. Usually the other terminal is ground, but you can also find them providing differential protection, like between cells of a battery charger.
These are typically found near connectors to prevent electrical nastiness from going inside your board and burning something sensitive.