Due to regulations about advertising on publicly owned wavelengths, one cannot place a 'direct response' ad on a public radio station (think of your local NPR station here). Instead, there is an 'underwriting message' allowed– typically shorter than a traditional ad, and with no direct sales pitch.
Usually somebody that buys podcast ads is also buying radio ads, and sometimes the terms they use get muddy. 'Underwrite' is probably the wrong term to use for podcast advertising, but since it's frequently used for public radio the meaning would be clear if you were in the industry.
The restrictions @mkmk is talking about apply to public radio stations, like NPR (and maybe some college stations).
The key distinctions are that underwriting has to be short (15 seconds each, 60 seconds total), non-competitive (you have to mention them all underwriters of a program), only at the beginning/end, and can't include a call to action. In other words, an underwriting message can say "This program brought to you by Crazy Eddie's Autos, purveyors of new and antique European cars" but not "COME ON DOWN to CRAZY EDDIE'S AUTO'S. WE'RE HAVING A BLOW-OUT SALE THIS WEEKEND ONLY blah blah"
Privately-owned stations can and do run either kind, however.
Due to regulations about advertising on publicly owned wavelengths, one cannot place a 'direct response' ad on a public radio station (think of your local NPR station here). Instead, there is an 'underwriting message' allowed– typically shorter than a traditional ad, and with no direct sales pitch.
Usually somebody that buys podcast ads is also buying radio ads, and sometimes the terms they use get muddy. 'Underwrite' is probably the wrong term to use for podcast advertising, but since it's frequently used for public radio the meaning would be clear if you were in the industry.