assuming you have at least one replica of the shard (i.e., 2 copies), when a node is stopped, Elasticsearch will try to allocate the shard somewhere else in order to maintain availability. Using delayed allocation, it will wait a while, hoping that the node that had a copy of the data comes back online. If it does, it will allocate it to that node. But if the delayed allocation timer times out, it will be allocated to another node (typically requiring a full recovery of all data from the primary copy of the shard).
We call all of those situations allocation (create index allocates an empty primary and 0 or more replica copies), a node that leaves the cluster can cause an allocation (if not using delayed allocation, shard will be allocated to another node), a node joining can also cause an allocation (if it has a copy and allocation is delayed) etc. Allocation assigns a shard to a node.