“主吉” in the context of traditional Chinese culture (like fortune-telling, feng shui, etc.) can be translated as "indicating good fortune" or "signifying auspiciousness".
If you want a more concise and direct translation, you could say "auspicious" (though "auspicious" is an adjective; to make it a phrase like "主吉" implies, you might say "a sign of auspiciousness" or "indicating auspiciousness").
For example:
此兆主吉。 (This omen indicates good fortune/auspiciousness.)
Translation: This sign signifies auspiciousness/indicates good fortune.