Like much Time Lord Tech, the Transduction Barrier(s) aren't really ever explained in cannon. Dr Who isn't a very technobabbly show; lost of tech "just works" or he'll "explain it later."
In the TV show, a transduction barrier is mentioned in The Invasion of Time and ...that's really it.
If you expand to accept the Webcasts as canon, then Shada mentions transduction barriers and some of their effects, and seems to pull from the Audio dramas as a "canon acceptable" source.
The Audio dramas provide a lot more background on the transduction barriers, including the concept that Gallifrey was kept "slightly out of phase" with the barriers, but in Dr Who's wibbly-wobbly "canon" universe, it's up to you to decide which bits are canon or not.
I would posit that the "Transduction Barrier" in the TV series is just a powerful planetary shield. The Time Lord's propensity for naming things with grandiose language would keep them from calling it a "planetary shield" or "border fence."
The word "Transduction" is used in many contexts to simply mean the conversion of one thing into another. In medicine, it means the conversion of one type of chemical or electrical signal in a cell to another. In Computer Science, it means converting one set of data directly into another via machine learning. I would therefore further posit that the transduction barrier is simply a fancy way of saying "electromagnetic force field;" it converts one kind of energy (electricity) into a (series?) of forcefield(s) which repel attackers, either as a pseudo-physical barrier, or simply a technological barrier (warp gravity and you can't achieve orbit or land. Put up enough static, you don't want to depend on your transmat beams.)
Finally, I would mention that New Who pretty much replaced "transduction barriers" with "sky trenches." Again, as above, no explanation is made on how a sky trench works, or how it can even be remotely trench-like, but these too appear to be a type of forcefield; until The Last Day, nothing had ever penetrated two layers of sky trenches. I would posit that "sky trenches," unlike the "transduction barriers," could also defend against time travellers. In Television Classic Who, there are very few species with Time Travel, with a heavy hint that the Time Lords were the only real people to perfect it, so they didn't really need special protections against time travel. In the Last Great Time War, however, at a minimum Daleks had perfected time travel for themselves, in which case transduction barriers would be "old tech" to be replaced with "sky trenches."
Putting Gallifrey into a time bubble and shifting it out of phase with the universe as a protection sure does sound cool, doesn't it? Kinda like stasis cubes...