An option is a contract that gives you the right - not the obligation - to buy or sell a stock at a set price (the strike) before a set date (expiration). A call is the right to buy; a put is the right to sell. Buy a call and you profit if the price rises above your strike; buy a put and you profit if it falls below.
What you actually pay for
You do not pay the strike price - you pay a premium, a small fraction of the stock's value, for the contract itself. That premium is the most you can lose as a buyer. It is also why options feel like leverage: a modest move in the stock can produce a large percentage move in the option. The trade-off is that the premium erodes over time (see theta decay) and reacts to changes in implied volatility.
Direction is necessary, not sufficient
The most common beginner surprise: you can be right on direction and still lose. If you buy a call and the stock drifts up too slowly, time decay can outrun your gains. If volatility falls after you buy, the option can cheapen even as the stock cooperates. Options price three things at once - direction, time, and volatility - and all three have to break your way, or at least not against you.
A call is not "the stock will go up." It is "the stock will go up enough, fast enough, before this contract expires."
Why this matters for short-dated trading
The shorter the expiration, the more brutal the math - which is exactly why 0DTE options are so unforgiving. The greeks that govern all of this (delta, gamma, theta, vega) are covered in our plain-English greeks guide. The takeaway for now: options are a precise instrument, and precision rewards discipline - a clean entry, a defined risk, and an exit rule set before you click.