If you are looking for a traditional encyclopedia explanation of the doctrine of the Trinity, you would be better off reading this:
If you think the Trinity is like an apple, where everything is the apple, just some parts of it are seeds, some parts are skin, some parts of meat... well, then you are in good company because the Missouri Synod publishing house publishes a children's book on that topic.
If you think the Trinity must be something like the mind, and want to spend time reading hundreds of pages exploring metaphors and analogies of the Trinity, then I suggest you make friends with Augustine and his psychological model of the Trinity.
And if you prefer to meet the Trinity as a social community in novel form, you still really can't beat The Shack.
But, if you want a very, very simple definition of the Trinity, one you can fit in a tweet or on a business card or memorize for personal use, I recommend two.
The first one is a definition: The Trinity is the way we tell the story of how Jesus Christ is both born of Mary and the Son of God, alive now in God and alive now also in us.
The second one is a liturgical action: The Trinity is the name (Hashem, Tetragrammaton) we speak over people at their baptism, and among the worshipping community when it gathers.