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.
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.