Half of the Israelites were on Mount Gerizim and half were on Mount Ebal. The cursings were on Mount Ebal, and the blessings were on Mount Gerizim. It is interesting that Moses had instructed Joshua to do this before the Israelites entered the Promised Land.
In Deuteronomy 8, the reasons for the blessings and cursings were given. God said there was the danger that after they were in the land that the many physical blessings such as land, silver and gold would make them forget God. They would be tempted to believe that they had gotten those things on their own.
Achan was deceived by the lure of riches at the very first city the Israelites conquered – Jericho. The defeat at Ai was a direct result of that sin. It was only after the nation of Israel made things right with God that they conquered Ai. This was a quick picture for Israel of the blessings and cursings. Immediately after the taking of Ai, they gathered at the mountains and reminded themselves of the blessings and cursings.
Today, we can have the blessings of the Christian life. As a Christian, we have the hope of the blessing of Heaven. If we do not live for Christ, we will neither have the spiritual blessings here nor will we have them in Heaven.
Study your Bible. Learn all you can from it. Obey God. If any of this is hard to understand, ask an adult to help you.