A PhD (Doctor of Philosophy) is the highest academic degree [certificate you get after finishing university] awarded by universities to individuals who have completed advanced research in a specific field of study [academic discipline].
For example: Someone who has earned a PhD in Psychology has completed extensive research and studies in the field of Psychology, and has written a dissertation [a long essay on a particular subject] based on their research.
Very Simple Definition:
A PhD is a fancy degree [qualification] that someone gets after studying and researching a lot in one subject [topic].
For example: If someone has a PhD in Math, it means they know a lot about Math and did a lot of hard work to get a special certificate [degree] that says they are really good at it.