The extract coming from the leaves and/or blossoms of Meadowsweet, a native to Europe plant that likes to grow in damp places such as meadows or the banks of streams.
It's main "skincare thing" is that it contains salicylic acid derivatives that give the extract anti-inflammatory properties. It also has tannins that make Spiraea Ulmaria Extract astringent, as well as spiraeoside that has anti-cellulitic properties.
https://incidecoder.com/ingredients/spiraea-ulmaria-extract