Jazakallah = May Allah reward you
This tells a person that you care about them because you are asking Allah to reward them
for what they have done for you.
You should say Jazakallah when someone is kind to you, helps you in any way or gives you something.
By saying Jazakallah to someone, you are asking Allah to give them more Thawaab so that they may go to heaven. In return, Allah also gives you more Thawaab. ]]>