Sentence:  Do n't forget - when you take to the hills or the beach - that your cooler , which you might have used for wine - or beer-cooling on your terrace or back yard , <WSD>is</WSD> indispensable for carrying liquid refreshments . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)