If you google the ASCII table it might make more sense but basically the characters 0-9 are grouped together on the ascii table. Meaning if you take the character of 0 ('0') and add 1 to it, you'll get the character '1' and its basically how this coder is converting int to char and then throws it into the string(which is an array of chars, c++ strings are bit fancier than that tho).

namespace is not needed and on more complex programs delcaring a golbal namespace can lead to naming conflits.