Think of an order pad in a restaurant. The waiter writes each order on a numbered ticket (the document and its _id), can cross it out to change a dish (partial update), tear it up (deletion), and each crossing-out is counted (_version). But the kitchen does not read the tickets continuously: it picks up the pile every few seconds. Between the moment the waiter puts the ticket down and the moment the kitchen picks up the pile, the order does exist (you can find it by its number) but it is not yet "in service". This periodic pickup is Elasticsearch's refresh, and it is the source of the most frequent misunderstanding among beginners.
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