Why did the laborers in the vineyard receive the same reward, and what does the murmuring signify?
The parable is an analogy for salvation. Some people become Christians (true believers in Christ) earlier in life than others, and some go through greater trials than others, and some live out the faith more consistently than others--so the Christians who do more are tempted to complain that they are getting an unfair deal--but God forgives us all and takes us all to heaven, and our works do not earn us this salvation at all. We should be happy that God extends his grace to anyone.