Sure. But first I'll tell you, integer division and remainders in Java arithmetic are covered well in the Wrox book Beginning Java. In Beginning Java Java 7 Edition
integer division and remainders are covered in chapter 2 "Programs, Data, Variables, and Calculation."
In Java, the standard division operator / returns just the whole integer part of the result. The modulus operator % returns the remainder. Using your example code and it helps me visualize this to pick a sample value for allSeconds of say 5000 to work through this
hours = allSeconds / 3600;
This gives you the whole number of hours in allSeconds. If the value of allSeconds is 5000: 5000 / 3600 yields 1
int remainder = allSeconds % 3600;
Now you're decalring remainder and getting the remainder of AllSeconds divided by 3600. With allSeconds at 5000, the % operator would leave 1400
minutes = remainder / 60;
Now the number of minutes again is the whole result of remainder divided by 60. Back to the sample, 1400 / 60 yields 23
seconds = remainder % 60;
And finally, the seconds are the remainder of your remainder divided by 60. So that 1400 / 60 had a leftover remainder of 20. That's the seconds.
Even though this is probably too late for your homework, hope it helps you or someone else.