A Broadband DC Block is a specialized form of capacitor. The capacitor is used in series within a Radio Frequency (RF) circuit to prevent DC current from flowing between two RF circuits while still allowing the RF signal to pass. The capacitor design is optimized to allow a wide range of RF frequencies to pass, making it a "broadband". They are further designed to minimize resonances across a wide range of frequencies, so they will not alter (distort) the RF signal as it passes through.
They can also be used as decoupling elements when connected between a signal line and ground. This is usually done as part of a DC bias network that is connected to an active RF device, such as an Amplifier.