主厨 is a verb-object compound (离合词) meaning 'to serve as the head chef' or 'to be the chef'. It is separable: you can insert time, aspect markers, or measure words between 主 and 厨, e.g. 主过厨 ('has been the chef'), 主了三年厨 ('was the chef for three years'). It is also commonly used as a noun to refer to the chef himself/herself, though the primary POS is verb-object. Use it in contexts of cooking leadership in restaurants or kitchens.