I am unable to understand this challenge. Please help me over here.

It's asking you to change the way the charge function is implemented. Currently, it just replaces the current charge with the max, but that isn't how it works in the real world. In the real world, charging take a while and should only occur when levels are low (Some batteries break if you over-charge them). The challenge is asking you simulate the real world by creating a loop that checks if its done charging. If it isn't, then increment the current charge level by 1.

The GoKart class has a handy method that returns True if the GoKart is fully charged. Because the charge loop should only run when the GoKart is not charge, we can use the ! operator in the while-loop's condition.

while(!isFullyCharged()){
    mBarsCount++; // mBarsCount = mBarsCount+1;
}

I hope this helps. Let me know if there is anything else I can clarify.