Another approach to Andy's fine answer:
Andy says, "The diode is used as a blocker".
A modulator can be made by thinking of the diode as a switch: it is in one of two states: open or else closed.
You can use a switch in a few different ways to either pass the baseband signal on to a load, or force the baseband signal to zero. It switches on/off at a rate determined by the carrier frequency, which is much higher than the baseband frequency.
Note that the baseband signal generator (V1, V2, below) has a peak amplitude of one volt added to a DC offset of one volt. So it swings between +2V on its top peak down to 0V on its bottom peak. This represents 100% modulation. With no AC peak amplitude at V1,V2, baseband would be steadily at +1V due to the D.C. offset:
simulate this circuit – Schematic created using CircuitLab
When you send the buffer's output on to an LC tuned circuit resonant at the carrier frequency of 25 kHz, the 1V offset is rejected, and the bottom half becomes inverse of top half.
If you were to send the output "out_1" to an antenna that can radiate a 25 kHz signal, the D.C. offset of 1V would not be radiated, but the 25 kHz carrier wave (along with its modulation) would be radiated.
However, harmonics of the 25 kHz signal would also be radiated, because it is squarely chopped. Using a 25 kHz resonator before passing to the antenna should reject harmonics, leaving the 25 kHz component (with its modulation) alone.
An electronic switch made from a FET transistor:
