Five letters scare more new options traders than any other concept. They should not. The Greeks are just the sensitivities of an option's price to four things that change while you hold it: the underlying, time, volatility, and rates. Get a working mental picture of each and the math stops being intimidating.

Delta - how much the option moves when the stock moves

Delta is the slope of the option price against the underlying. A 0.40 delta call gains roughly $0.40 if the stock moves up $1. It is also the rough probability the option finishes in the money. ATM options sit near 0.50 delta. As you go deeper in the money, delta climbs toward 1.0. Way out of the money, it approaches 0.

Gamma - how fast delta changes

Gamma is the curvature. It is highest right around the strike and rises sharply as expiry approaches. High gamma is what turns a quiet 0DTE position into a violent P&L swing - your delta is shifting in real time.

Theta - the cost of time

Every day an option exists, a piece of its time value bleeds out. Theta measures that bleed. A long option pays theta. A short option collects it. Theta is non-linear: it is small with 60 days left, brutal in the last 14.

Vega - sensitivity to implied volatility

If IV expands by one point, vega tells you how much premium you gain (if long) or lose (if short). Vega is highest in ATM, mid-dated options. This is why selling premium into earnings is so dangerous: vega works against the short side as IV spikes.

Rho - sensitivity to interest rates

Smallest of the five for the typical retail position. Rho only matters when you are running LEAPS or sizable books exposed to rate moves.

Why this matters when you read flow

When you see unusual call buying, the trade you are reading is not just bullish. It is bullish, long vega, long gamma, and paying theta. Knowing that, you understand why a holder wants a sharp move soon, not a slow drift. The Greeks tell you the trade's clock.