In one sentence
Svadharma is your own dharma — the specific right way of being that is yours alone, shaped by your nature, your stage of life, and your responsibilities.
The depth — what it actually means
The Gita's most personally radical concept. Verse 3.35 and verse 18.47 both teach that one's own dharma, even imperfectly performed, is better than another's dharma well performed. The point is that there is no universal template you can apply to yourself. Your right life is shaped by who you actually are. Trying to live someone else's right life — even an admirable person's — is a form of self-betrayal.
Modern application — how to use this today
When envious of someone's path or tempted to imitate it, return to svadharma. Their right life is not yours. The work fitted to your nature, even when imperfect, is the only work that will be sustainable over decades. The work fitted to someone else's nature, even when well performed, will eventually break you.