I have only heard this in relation to perfume and scents. Soap makers, or perfume makers, buy various essential oils and mix them in different ways to make their own scented products. That;s big business, by the way. One half liter of an essential oil can go for thousands of dollars, because it is diluted so much to go into the various products. There are probably other meanings for ‘essential oils’, but this is the one I am familiar with.