Dactylic pentameter is a way of writing poems that sounds like a bouncy song when you read it out loud. Imagine bouncing a ball on the ground, but instead of saying "bounce" each time it hits the ground, you use syllables that sound like "dadadum."
So a line of dactylic pentameter might go: "In the gar-den of beau-ty and light." See how there are five sets of "dadadum" sounds in that sentence? That's what makes it dactylic pentameter.
It's a special way of counting syllables to make sure each line of the poem has the same rhythm and bounce. Some poets love to use dactylic pentameter because it sounds fancy and musical, while others prefer other types of rhythms. It's all up to personal preference!