Energy let's you work in your farm and around town. Every time you use a tool, your energy goes down. The amount of energy used varies from tool to tool, and may be affected by your proficiency level.
You can consume certain fruits and food that may restore your energy.
When your Energy reaches 0, you become exhausted, and faint, walking up the next day in your bed. If you go to bed exhausted, you wake up the next day already tired. Your energy may be at less than 100%.