A relative clause is a clause that starts with <em>a relative pronoun</em>, and provides more information about the noun that precedes the clause. The most common relative pronouns are<em> who, whom, whose, which, and that</em> ( but <em>where</em> is also sometimes used as a relative pronoun).
In the sentence <em>'Last Friday was the day that Karl won the marathon', </em>there is a noun - <em>day</em>, followed by a relative clause<em> 'that Karl won the marathon'</em>, which gives us more details about that particular day.