In plain words
Heroism, boldness, firmness, skill, not fleeing from battle, generosity, and the capacity to lead are kṣātra-Karma born of Svabhāva.
Word for word
Valour, boldness, firmness, skill, and also not fleeing in battle, giving and lordliness: the duty of the kṣatriya, born of svabhāva.
The commentators
Bhagavadgītābhāṣyam· bhāṣya
शौर्यं शूरस्य भावः, तेजः प्रागल्भ्यम् , धृतिः धारणम् , सर्वावस्थासु अनवसादः भवति यया धृत्या उत्तम्भितस्य, दाक्ष्यं दक्षस्य भावः, सहसा प्रत्युत्पन्नेषु कार्येषु अव्यामोहेन प्रवृत्तिः, युद्धे चापि अपलायनम् अपराङ्मुखीभावः शत्रुभ्यः, दानं देयद्रव्येषु मुक्तहस्तता, ईश्वरभावश्च ईश्वरस्य भावः, प्रभुशक्तिप्रकटीकरणम् ईशितव्यान् प्रति, क्षात्रं कर्म क्षत्रियजातेः विहितं कर्म क्षात्रं कर्म स्वभावजम् ॥ ४३ ॥
In plain words
Śaurya is simply what it is to be a hero. Tejas is boldness. Dhṛti is steadiness — the very thing that props a person up so he does not collapse, whatever the circumstance. Dākṣya is competence — acting clearly, without confusion, the instant some urgent task lands on you. Not fleeing in battle means never turning your back on the enemy. Dāna is an open hand toward whatever is meant to be given. And īśvara-bhāva, lordliness, is simply what it is to be a master — visibly exercising that authority over those in your charge. Kṣātra karma is just the duty laid down for the kshatriya class: kshatriya-duty, born of his own nature.
Close to the Sanskrit
Śaurya means the state of being a hero; tejas means boldness; dhṛti means steadfastness — that by which, being propped up, one does not sink in any condition; dākṣya means the state of being capable — engagement without confusion, suddenly, in tasks that have just arisen; and also, in battle, not fleeing means the state of not turning one's face away from enemies; dāna means open-handedness with regard to things to be given; and īśvara-bhāva means the state of being a lord — the manifesting of one's power as master toward those who are to be ruled. Kṣātra karma means the duty prescribed for the kshatriya class — kshatriya-duty, born of its own nature.