<div class="img_div">
    <img src="./images/paella-dish.jpg">
 
        <a href="#">
            <div class="mask">
               <h3>A Picture of food</h3>
                </div>
        </a>
</div>

.img_div {
    margin: 20px 400px 0 400px;
    position: relative;
    width: 531px;
    height: 354px;
}
.mask {
    position: absolute;
    top: 0;
    left: 0;
    width: 531px;
    height: 354px;
    background: rgba(101, 101, 101, 0.6);
    color: #ffffff;
    opacity: 0;
}
.mask h3 {
    text-align: center;
}

.img_div a:hover .mask {
    opacity: 1;           
}