4 ideas
On the battlefield of Kurukshetra, the warrior Arjuna looks across at the opposing army and sees his cousins, his beloved grandsire Bhishma and his teacher Drona. He drops his bow and tells his charioteer Krishna that he would rather be killed than fight, and Krishna answers not by ordering him but by teaching him why to act. That conversation, held between two armies just before the war begins, is the entire Gita.
Krishna tells Arjuna that he has a right to his actions but never to their fruits. The mechanism is to put all your effort and attention into doing the work well and to give up any claim to how it turns out. Anxiety about results distorts the action and chains you to success or failure, while letting go frees you to act fully.
The Gita argues that it is better to do your own svadharma imperfectly than to do someone else's duty well. Your duty comes from your particular role, nature and situation, so borrowing another person's path, even a nobler-looking one like Arjuna's wish to become a renunciant, is a form of evasion. Your worth is measured by fidelity to your own role, not by comparison with others.
Krishna describes all of nature, including human minds, as a changing mix of three qualities. These are sattva (clarity, balance), rajas (restlessness, ambition, passion) and tamas (inertia, dullness, confusion). Looking at food, work, charity or a mood through this lens shows which quality is driving it, so you can deliberately build up sattva instead of simply reacting.