please explain the code>

rosinit
sim = ExampleHelperRobotSimulator('complexMap');
setRobotPose(sim, [4 2 -pi/2]);
enableROSInterface(sim, true);
sim.LaserSensor.NumReadings = 50;
scanSub = rossubscriber('scan');
[velPub, velMsg] = rospublisher('/mobile_base/commands/velocity');
tftree = rostf;
pause(1);
path = [4.5, 2; 3.5 4; 8 5.75; 1 10; 1 14; 9 15; 5 18; 9 15; 6.5 12; 9 15; 14 15; 12 18; 14 15; 13 12; 14 15; 16 15; 16 12; 16 15; 20 19.5; 25 16; 20 9;
15.5 5.5; 25 4; 16 4; 16 1; 16 4; 13.5 4; 13.5 2; 13.5 8; 13.5 4; 8 4; 8 2];
plot(path(:,1), path(:,2),'k--d');
controller = robotics.PurePursuit('Waypoints', path);
controller.DesiredLinearVelocity = 0.4;
controlRate = robotics.Rate(10);
goalRadius = 0.1;
robotCurrentLocation = path(1,:);
robotGoal = path(end,:);
distanceToGoal = norm(robotCurrentLocation - robotGoal);
%pyenvmap = robotics.OccupancyGrid(14,13,20);
map=robotics.OccupancyGrid(25,20,20);
figureHandle = figure('Name', 'Map');
axesHandle = axes('Parent', figureHandle);
mapHandle = show(map, 'Parent', axesHandle);
title(axesHandle, 'OccupancyGrid: Update 0');
updateCounter = 1;
while(distanceToGoal>goalRadius)
scan = receive(scanSub);
pose = getTransform(tftree, 'map', 'robot_base', scan.Header.Stamp,'Timeout', 2);
position = [pose.Transform.Translation.X,pose.Transform.Translation.Y];
orientation = quat2eul([pose.Transform.Rotation.W,pose.Transform.Rotation.X,pose.Transform.Rotation.Y, pose.Transform.Rotation.Z], 'ZYX');
robotPose = [position, orientation(1)];
ranges = scan.Ranges;
angles = scan.readScanAngles;
ranges(isnan(ranges)) = sim.LaserSensor.MaxRange;
insertRay(map, robotPose, ranges, angles,sim.LaserSensor.MaxRange);
[v, w] = controller(robotPose);
velMsg.Linear.X = v;
velMsg.Angular.Z = w;
send(velPub, velMsg);
if ~mod(updateCounter,50)
mapHandle.CData = occupancyMatrix(map);
title(axesHandle, ['OccupancyGrid: Update ',num2str(updateCounter)]);
end
updateCounter = updateCounter+1;
distanceToGoal = norm(robotPose(1:2) - robotGoal);
waitfor(controlRate);
end
show(map, 'Parent', axesHandle);
title(axesHandle, 'OccupancyGrid: Final Map');
rosshutdown
displayEndOfDemoMessage(mfilename)

[Sudhakar Shinde]

check this example:

https://www.mathworks.com/help/robotics/ug/mapping-with-known-poses.html

[Rik]

Have a read here and here. It will greatly improve your chances of getting an answer.