The main thing you don’t understand is std::pair http://www.cplusplus.com/reference/utility/pair/ which combines two types (or two of the same type) into the type of one object made from those two.
Also notice the using namespace std; that lets the programmer leave off the std::. That is poor style for many reasons and you shouldn’t do it yourself, but should understand when others do.
so std::pair<int, int> is a type. Objects of that type consist of two ints the first of which is named first and the second named second. So it is a type very similar to:
struct { int first; int second; };
If cur had been an object of the above kind of struct, I assume you would understand what cur.first and cur.second and if you knew what cur.first is then you would know what cur.first-1 is (no magic meaning, just computing a value one less than the value of cur.first
And, yes, that code is finding a path between two points on a grid.
The programmer chose to represent a position {row,column} as a pair, which is a bit lazy. Then compounded that by failing to comment the fact that the pair<int,int> is used to hold {row,column}
So all those >0 and <4 checks are ugly ways of testing whether a specific neighbor of the current {row,col} really exists (as opposed to being off the edge of the map).
Next you’ll want to read about std::queue http://www.cplusplus.com/reference/queue/queue/
One of those is used to hold all the reachable {row,col} pairs for which the distance from {0,0} has been computed but for which the distance from {0,0} of some neighbor might not have been computed. So the basic operation is take one of those from one end of the queue; check all of its neighbors; and for any neighbor whose distance is newly discovered, insert in the other end of the queue.