A yoyo is a toy made from two heavy halves with a gap between them. A string is looped around an axle in the gap, but not tied on. You hold onto the end of the string, usually by a loop that goes around your finger, and you can make the yoyo go up and down. You can also do lots of other tricks once you get good at playing with a yoyo.
Yodelling is a kind of singing that frequently changes between your ordinary voice and falsetto. Swiss mountaineers sing like this.
Yoga is a traditional mental and physical discipline originally from India. In modern times in the Western world, yoga usually takes the form of poses and stretches as a form of exercise, but traditionally was also an aid to meditation.
Yoghurt is made from fermenting milk. Yoghurt is very sharp and tangy on its own. Modern yoghurt often has fruit and sugar added. Yoghurt is nice to eat on its own, with fruit or breakfast cereal, or frozen as a kind of icecream.
A yoke is something that connects two oxen together at the neck so they can pull a heavy load. Yoke also refers to the part of a garment that goes around the top part of the shoulders, front and back, around your neck.
The yolk of an egg is the yellow part inside the egg. The clear or yellowy-white part is called the white of the egg. It goes white when you cook it, and the yolk stays yellow but goes crumbly.
You is a single person being addressed. When you talk about yourself you usually say I or me. When you talk about someone else and you are talking to them, you say you. For example, you are reading this page.