One becquerel is a level of radioactive decay where one atom is decaying every second on average. This is a really low number and the naturally occurring radioactive elements in your body means the whole of your body has a natural level of many thousands of becquerels anyway. Nevertheless, the LDR is going to increase that by probably a million times in the prostate. 1.67GBq is 1,670,000,000 atoms decaying per second and giving off their radiation as a result.
The becquerel is not a very useful unit for treatment effect for a several reasons...
It says nothing about the energy of each radioactive decay, which vary over an enormous spectrum between different isotopes of different elements. Brachytherapy uses low energy radiation so it doesn't travel far in the body (I think not much more than 1cm), and the treatment is thus limited to the area around the seeds. The energy of each decay for Iodine125 is 35keV (kilo electron volts), although mostly from an intermediate decay product. In contrast, the energy of the radiation in external beam which has to pass through much more tissue than 1cm to even reach the prostate is 6MeV, which is 170x more energy per photon. (It would in theory be possible to work out the equivalent becquerels for external beam, but I've no idea what the beam intensity is in terms of photons/second.)
A more useful unit for treatment effect is the Gray (Gy), and this is a measure of the radioactive energy absorbed by a kg of tissue. A prostate LDR brachy is usually 150-170Gy in the case of using Iodine125. This is a high figure compared with any of the other radiotherapy treatments (EBRT 60 or 74Gy, SABR 37Gy, HDR 32Gy) because more energy is required for the same treatment effect the longer the treatment takes, and it's about 200 days for LDR brachytherapy using Iodine125 before the rate of radioactive decay in the seeds drops too low to be treating any more.