Within the outboard structure of each warp nacelle which house the ship's warp engines, there is a Bussard Collector. It is located at the front of the warp nacelle. Also known as a Bussard ramscoop, the Bussard Collector employs powerful electromagnets to attract interstellar hydrogen ions for use as fuel by the ship's fusion reactors. The concept was developed in the 1960s by real-life physicist Robert Bussard.

Nacelle detail, including Bussard collector and part of the photon radiator grille
Source: STMech

Location and extent of radiator grille

Bussard Collectors can also be used as a weapon e.g. in [#108 Bliss] Voyager was trapped for a time inside a telepathic "pitcher plant": the ship collected gas using the port-side Bussard Collector then expelled it through the port-side radiator grill, and on ignition of the gas the entity felt that Voyager "tasted" bad and regurgitated the ship.