You have to make a trip to Australia ahead of the expiry of the first entry date sequentially to activate the visa for its full time which, generally, is a full term of 5 years (the date is on the visa label. Migrant visa holders are not necessary to migrate (i.e. stay permanently) to Australia before the expiry of the first entry date on their visas. Provided you have made no less than one trip to Australia ahead of the initial entry date your visa is then legitimate for return travel until the final termination date.
There are exceptions to this common rule. For instance, Spouse - Provisional visas (subclass 309) and Prospective Spouse visas (subclass 300) together have first entry dates but neither is legal for a full term of 5 years. Business skills visas also cover definite conditions close to them.
