This passage refers to that experience in the history of Israel when they were in the wilderness at the edge of the "Promised Land." This event is recorded in Numbers chapters 13-14. When Israel had the chance to go into Canaan, which is the Promised Land, they wouldn't believe that God would keep them safe there. They were afraid their "little ones" would be killed by the people of that land. God was angry with their unbelief and basically said that the older generation would never enjoy that new land, but He would allow their children to win the battle against the Canaanites which is exactly what happened as recorded in the book of Joshua.
God wants to be believed! And He rewards those who put their faith in Him. Another PPIL (Powerful Passage Impacts Life) verse that I'll comment on sometime soon is Hebrews 11:6 - "But without faith it is impossible to please Him, for he who comes to God must believe that He is, and that He is a rewarder of those who diligently seek Him." God speaks and He expects us to believe what He says! And when we find Him faithful we should have no reason to disbelieve Him!
Now, what about "doubts?" Is "doubting" a sin? The answer is "yes" and "no." Its "YES" if the doubt is masking the real issue: unbelief. If you know the facts, God wants your faith in Him. The answer is "NO" if the problem is lack of knowledge. Some people have honest doubts simply because they lack the knowledge of what God says. But, when they get that knowledge and continue to doubt, God calls that "Unbelief!"
God has so much to do in our lives if we would just believe Him!