PHP 中的运算符 “? :“ (三元表达式)有什么作用？

 @media screen and (max-width:801px) {
                    .ff-container {
                        padding: 10px;
                    }
                    .ff-container .inner {
                                                 grid-column: span 12;
                    }
                    .ff-container .inner h1{
                        font-size: 18px;
                    }
                    .ff-container .side {
                        grid-column: span 12;
                    }
                }