Its when you do HRT without a medical professional. This means that people buy (often through the gray/black market) and administer their hormones themselves and monitoring the hormone levels by themselves It may sound like something that is very sketchy and dangerous, but it is not as bad as it sounds. There are lots of high quality ressources out there that allow you to find out everything you need in order to safely do HRT (some additional ressource will be linked below). The biggest problem is that it is quite sketchy to buy hormones through the gray market with crypto from some person that you have to trust with not fucking up your hormones, but there is also quite a big network of organizations testing the samples from known producers. Lots of producers also provide lab tests by themselves. Additionally there are big communities around DIY HRT (e.g. Reddit has a very big community around DIY HRT) and if contaminated doses appear, than this spreads quite fast in the community. So far I am not aware of any bigger scale of contaminated or bad batches of DIY HRT, that came from reputable sellers.

Something that should also be considered, that often enough medical professionals also dont know that much about HRT, which often leads to unoptimal or straight up dangerous dosages, which means that in some cases its literally much more safe to do HRT on your own means than by trusting an untrained doctor that gives you ineffective and straight up dangerous dosages.

For people who want to consider taking estrogen I can highly advice visiting https://transfemscience.org/. It is the best website I know if you want to gather a fundamental understanding of how HRT works and what you need to watch out for.

https://diyhrt.info/ also has some great ressources and https://transharmreduction.org/ is very active in providing harm reduction through testing of HRT medications and providing guides for various things like how to inject medications or on blood tests (very important if you want to do DIY HRT).