From Wookieepedia, the Star Wars wiki.

Ion Limpet homing beacon
Production information
Model	Ion Limpet
Type	Homing beacon
Usage and history
Era(s)	Rebellion era
Affiliation	Boba Fett

[Source]

The Ion Limpet homing beacon was a device used to track starships through space. It had a disc like appearance and its thin profile helped prevent it from being detected. It utilised the HoloNet to track spacecraft through the known galaxy.

The bounty hunter Boba Fett used a Ion Limpet homing beacon.

Sources

• The Complete Visual Dictionary