Positive feedback is a technical term for what is described in everyday language as a "vicious circle". It is a situation where the result of a process contributes to a strengthening of the original causes of the process, which in turn causes the result to become even larger, again strengthening the process, etc.
"Positive" is used in a mathematical sense, implying that the effect of the process goes back and influences the cause in a positive (increasing) sense, making it bigger (instead of diminishing it.) This use of the word "positive" can be confusing, because in everyday language, there is nothing "positive" about "positive feedback" (in other words, it's not a "good" thing).
An example is when you are in a place with a microphone amplifying a voice through speakers. If the microphone gets too close to the speakers, it begins to amplify the output of the speakers, back through the speakers, in an increasingly louder and ever louder shriek. That horrible sound is commonly just called "feedback".