Now a days, i am hearing too much news and promotions about Decentralised crypto systems, wallets and exchanges that ''these upcoming currencies and exchanges are secure then ever''. What does it mean? how actually do these projects work without any central command or supervision/server.

In case of any error or reconciliation who would help us, if creator himself having no power/command/access?
Is Decentralisation or Dapps just secure or trouble free as well?