Previous posts are well worth a full read, but one key answer; a (true) stack turns the units into one virtual switch, from the point of view of STP (and other features, such as Link Aggregation).
Note this does not apply to Meraki Virtual Stacking, which is not really stacking, in the networking sense - it just provides the power of having all your ports configurable in one place for a whole Network (which is one benefit of true stacking.)