First, this is horrible code that should be refactored immediately and the original programmer slapped with a dead mackerel.
Second, what happens with that code is this (at least in theory - a smart compiler should optimize this out):
The value contained in the 'i' variable is incremented. However, the expression 'i++' evaluates to the pre-increment value of 'i'. The '=' assignment operator has the least precedence, so it happens last: the pre-increment value of 'i' is assigned back to 'i', effectively leaving you with the original value of 'i'.