Earnest can either be used as an adjective or noun. Earnest as an adjective can mean that you are serious or sincere about something. Earnest as a noun can mean something of value given to someone to serve as a contract.
An example sentence using the word earnest as an adjective could include: "Your parents might not want to drive you to all your baseball practices and games, but if you're in earnest about playing then they will support you."
Another example could be "Bobby is a very earnest student because he completes all his homework and tries really hard to participate in class."