An emulsifier is a substance you add so a mix does not split. The classic job is oil and water: they do not want to stay together, and an emulsifier helps them stay as one smooth mix. Food people also use it so a sauce or ice cream does not separate — liquid and fat, or liquid and bits that would settle. You will see it on packets (things like lecithin). In the kitchen, egg yolk and mustard can do this job. It is not a flavour; it does not make the food taste of something new.