Countries that have an alliance, or agreement, help each other out when in need. That means during times of war, for example, if two countries have an alliance, one will help the other by providing weapons and supplies.
An alliance doesn't just have to be between two countries; you can see alliances formed on a playground, when kids start clustering in groups or dividing themselves into teams to play tag. An alliance is essentially a connection through which a common interest is shared. A marriage is an alliance between two people who decide to share their lives together.