This is a branch (no pun intended) of phytotherapy that employs remedies made from the tissues of a plant that are involved in plant growth; typically the buds ("Gemmo-" from the Latin for bud), but parts of the roots and stems could be used too. These tissues contain undifferentiated cells, that is, ones that haven't developed into specialized tissues like those found in fruits and flowers. They have the potential to transform into any part of the plant, i.e. plant stem cells. Areas with this kind of cells in abundance are known as the meristem.
Chemical analyses have shown that these tissues contain high concentrations of a wealth of phytochemicals, which are normally found at much lower levels throughout mature plants, due to their role in increasing the size and length of a plant. The preparations of these extracts therefore energetically have a more anabolic nature and may be better suited to states where the body needs to grow, repair and reproduce. Gemmotherapy is somewhat more common in continental Europe than other places, and more research has subsequently been done there.
One study by Donno et al. 2016 showed that the concentrations of phytochemicals is highest at the bud break stage in Ribes nigrum and Rubus spp. A useful work on the topic is by Nicoletti and Piterà di Clima's Gemmotherapy, and the Scientific Foundations of a Modern Meristemotherapy. I mention the connection between gemmotherapy and spring in an episode of The Red Thread podcast, as well as a general introduction of myself here. I also mention Gemmotherapy in an instagram video here.
Donno et al. 2016