Ethyl Maltol is a sweetener used in many e-liquids - it is not a very strong, but can be very helpful in making a bland flavour shine. Too much however can mute some flavours, so it is recommended not to use more than about 4% in a mix.
Besides sweeting, EM can also round out and smooth those flavours that has very sharp or rough notes to them, as well as adding to the 'fullness' feeling of the vapour in your mouth.
This is a 10ml bottle filled with EM crystals: Recommended is to dilute 1 quarter teaspoon into 10ml pure PG , so this 10ml bottle will go a very long way.
NOTE: If the crystals are not diluting in your PG mix, heat slightly by running warm water over it for 10 mins. If the resulting sollution gets very cold, the EM could re-crystalize, simply re-heat to dissolve them again and always shake before using.