To earmark something is to set it aside for a specific purpose. If you’re saving money to spend it in a particular way — whether it’s for college or a fancy new pair of shoes — you have earmarked that money.
Originally, an earmark was a mark on the ear of an animal — such as a sheep — that indicated ownership. Similarly, when government officials earmark a resource, they're indicating what the resource will be used for. Earmarking is usually about money. $10,000 could be earmarked for cancer research. A grant could be earmarked for a children's hospital. Earmarking is a way of stating how something will be used.