An option is at-the-money when it's strike price is equal to the price of the underlying. An at-the-money option has no intrinsic value since the strike price and the stock price are the same.
To understand this, it's important to identify how we get to the intrinsic value of an underlying: Strike price - underlying price = intrinsic value. For example, if the strike price is $150 and the underlying price is $148.60, then it's intrinsic value will be $1.40. At-the-money options, therefore, have no intrinsic value.