sqrt( ((x2-x1)^2) + ((y2-y1)^2) )

... Which could be wrote as:

a = (x2-x1)
b = (y2-y1)

sqrt( (a*a) + (b*b) )

... Which is essentially Pythagoras' Theorem, which finds the length of the hypotenuse of a right angled triangle.
Because we know two sides (x and y), we can use these to calculate the length (or distance between) the two points.

stock Float:dif_between_coords_xy( Float:x1, Float:y1, Float:x2, Float:y2 )
{
    //Formula to calculate difference between two sets of cartesian coords:
    //sqrt( ((x2-x1)^2) + ((y2-y1)^2) )
    
    new Float:a = (x2 - x1),
        Float:b = (y2 - y1),
        Float:distance;

    //Assign distance the result of the formula.
    distance = floatsqroot( floatpower(a,2) + floatpower(b,2) );

    //Return distance.
    return distance;
}

//Last position X:
new Float:LastPosX [ MAX_PLAYERS ];

//Last position Y:
new Float:LastPosY [ MAX_PLAYERS ];

//Total distance travelled:
new Float:tDistance[ MAX_PLAYERS ];

public OnPlayerUpdate(playerid)
{
    //Variables for the player's current position:
    new Float:fX, Float:fY, Float:fZ;

    if( IsPlayerInAnyVehicle( playerid ))
    {
        //If the player is in any vehicle, get the vehicles position:
        GetVehiclePos( GetPlayerVehicleID(playerid), fX, fY, fZ );
    }
    else
    {
        //If the player isn't in any vehicle, get the players position instead:
        GetPlayerPos( playerid, fX, fY, fZ );
    }
    
    if( LastPosX[ playerid ] != 0 && LastPosY[ playerid ] != 0 )
    {
        //If the player's last position X&Y are not 0:
        //(They've been initialised):
        
        //Add the difference between the two sets of coordinates to the distance travelled.
        tDistance[ playerid ] += dif_between_coords_xy( fX, fY, LastPosX[ playerid ], LastPosY[ playerid ] );
    }

    //Set the last position variables to the player's current position, for the next update.
    LastPosX[ playerid ] = fX;
    LastPosY[ playerid ] = fY;
    
    return 1;
}