Most options courses dump all five Greeks on you in week one. The Express Lane teaches only two: delta and theta. Master those and you can place 90% of beginner trades intelligently. Vega, gamma, and rho can wait.
Delta is the option's sensitivity to a $1 move in the underlying. A 0.40 delta call gains roughly $0.40 if the stock rises $1. Delta is also a rough proxy for probability of finishing in-the-money: a 0.30 delta option has approximately a 30% chance of being ITM at expiration. That second meaning is the more useful one for strategy selection.
Theta is the option's daily premium decay. A position with -0.05 theta loses $5 every day the underlying does nothing (per contract). Long options have negative theta — time hurts you. Short options have positive theta — time pays you. That single distinction tells you why selling premium and buying premium are fundamentally different psychological games. Everything else is detail.
Frequently Asked Questions
When do gamma and vega start to matter?
Gamma matters when you hold short options into expiration week. Vega matters when you trade volatility strategies (straddles, strangles, condors) or trade earnings.
Is delta exactly equal to probability of ITM?
Approximately, not exactly. Delta is mathematically a hedge ratio; the probability interpretation is a useful shortcut that overstates ITM probability slightly for OTM options.