Ketoembar is the Dutch spelling of the Indonesian word for the spice coriander, and in particular ground dried coriander seed, which plays an important role in Indonesian cooking. Indian, Malaysian, and Burmese cuisines also feature ketoembar/coriander in this form. Laotian, Vietnamese, and especially Thai food tend to use fresh coriander (leaves, stems, and even roots) instead.
For lots more about this spice and its uses, see sneff's excellent writeup on coriander.