What does by faith Abel offered unto God a more excellent sacrifice than Cain by which he obtained witness that he was righteous God testifying of his gifts and by it he being dead yet speaketh means?

It just means that God asked Cain and Abel to give him sacrifices as an act of worship. Both Cain and Abel knew what kind of sacrifice God wanted - but only Abel gave the right kind. Cain (on principle because he was proud and rebellious) refused to offer God an acceptable sacrifice. Abel -was prompted by his faith in God (that is he truly understood who God was) and as such was careful to give God a sacrifice that was acceptable. Cain (full of rebellion and pride) gave what he wanted to give God not what God wanted. One sacrifice was pleasing to God the other was not - and the pleasing sacrifice of Abel - given from a right heart with a right understanding of who God is - caused God to see him a righteous. The example that Abel set by responding by faith and the righteousness that he gained through that act of faith - still speaks powerfully (witnesses) to us today about what it is that God requires from every person. It is an example of how we can be made righteous with Him on the basis of faith in Him - but that this faith always produces actions that are pleasing to God. It is those right actions which are proof that we have true faith - The apostle Paul wrote "faith without works is dead". (But also works without faith are dead. Kinda like advertisign without a company, or vice versa. How are you going to achieve.)