Millet is a kind of grain which is gluten-free so in that sense, it's similar to rice but more nutritious. It doesn't taste or smell nice on its own and you must cook it or use boiling hot water to get a creamy texture.
You can find it in the organic foods section - I buy mine organic and ground into flour already.
I usually mix some barley and fruit/veg purees with millet to mask the yucky taste; Eva will finish it anyway.