Each line on the birth chart represents an aspect or relationship between two planets. Some are in harmony, and this is where you'll have natural talents, ease or good fortune.
A trine is between planets in signs of the same elements -- air, water, fire or water. They're also spaced every fourth house apart. Find the trines in your own chart, and contemplate what it could mean based on your personal experiences.
I've highlighted here another example of a trine in Harry's chart. Harry's Gemini Mars makes a trine (harmony) to his Moon in Libra since they are both Air signs. With this favorable aspect, his emotional instincts and the manner in which he takes action will blend well together.