Thank you so much for playing!
It's not noticeable at lower levels, but Tribute boosts the card stats overall. The stronger the card, the more you'll notice the boost, but it doesn't matter at what level you do it, it stays forever. It can also be stacked up to three times.
And on the other hand, the card you got the Tribute from gets a little "x" mark at the top, meaning it gets weaker and it can't be upgraded anymore. Hope that helps!