To gladden is to make someone happy by delighting them or cheering them up. It will gladden your heart to finally find your lost dog.
The verb gladden means "to make glad," from the Old English glæd, "bright, shining, gleaming" and also "joyous." So when something gladdens you, like running into your beloved kindergarten teacher, it brightens your whole day. Before the early 14th century, people used the word glad as a verb instead, as in: "It glads my ears to hear such beautiful music."