Position caption over image and show only on hover.

envira-caption-over-image.css

/* Position caption and hide it by default */
.envira-gallery-item-inner .caption {
   opacity: 0;
   position: absolute;
   top: 50%;
   left: 50%;
   transform: translate(-50%,-50%);
   
   /* Optional: add transitions so the caption smoothly fades in/out on hover */
   -webkit-transition: opacity 0.5s ease;
   -moz-transition: opacity 0.5s ease;
   -o-transition: opacity 0.5s ease;
   transition: opacity 0.5s ease;
}

/* Show caption on hover */
.envira-gallery-item-inner:hover .caption { opacity: 1; }