Foils is (are?!) THE highlighting technique of the moment (in fact, it's prob a bit outdated now. It's when people with dark hair suddenly get these blond sections. Blonds often get foils, because it makes the blond look more 'natural', less of an all over dyed effect. The hairdresser applies the highlights to sections, wherever they want to create a particular effect, covers the hair with foil, and voilà, you have your 'natural' looking effect! It's 'libre' because the hairdresser can choose to just dye the fringe, for example, or can dye large or small sections ...

to support my idea below (no mention of the swim cap though!) :
Foiling: Small sections of hair are placed onto rectangular sheets of foil and color or lightener is applied, and the foil is folded to keep color in place. Closest application to hair root of all highlighting techniques.

Baliage: Haircolor or lightener is "handpainted" freestyle onto select strands of hair; great for personalizing color on textured or wavy hair.

add streak highlights to hair means, to add lines of one or more colours to your hair for more luminosity (brightness) .