The % operator returns the remainder of the division calculation.
In this instance 10 goes in to 2 zero times and has a remainder 2.
If you did 5 % 2, the value would be 1 as 2 goes in to 5 2 times and has a remainder of 1.
WIth regards to your initial question:
std is a class which is contained in the C++ libraries and cout is contained within that class. [I have chosen cout for this example but the same principal applies to everything that uses the std class]
Therefore, every time you want to use cout you need to write:
By including "using namespace std" you can now just write
cout and don't have to add std:: everytime you use cout.