The purpose of a port probe in a load-balanced set is for the load balancer to be able to detect whether or not a VM is able to accept traffic. When configuring the load-balanced endpoint you can specify a webpage or a TCP endpoint for the probe - and this should be present on each instance. Traffic will be directed to the VM as long as the webpage returns 200 OK or the TCP endpoint accepts the connection when the load balancer probes. You can specify the time interval between probes and the number of probes that must fail before the endpoint is deemed dead and should be taken out of rotation (defaults are every 15 seconds and 2 probes).
You can take a VM out of load-balancer rotation by ensuring that the configured probe page returns something other than 200 OK and then bring it back into rotation by having it once again send a 200 OK.