Complex sentence: A preselector is a name for an electronic device that connects between a radio antenna and a radio receiver, that blocks trouble-causing out-of-tune frequencies from passing through to the radio receiver or preamplifier that otherwise would be directly connected to the antenna.
Split into simpler sentences: A preselector is a name for an electronic device that connects between a radio antenna and a radio receiver. The preselector blocks trouble-causing out-of-tune frequencies from passing through from the antenna into the radio receiver (or preamplifier) that otherwise would be directly connected to the antenna.