Unlocking a phone from network has nothing to do with manufacturer updates. Its more of, being able to take a SIM card from another network and pop it in.
For example, buy a handset from Orange, and using a T-Mobile SIM card in the new Orange handset, the T-Mobile will not work as the handset is locked into the Orange network and thusly refuse any other SIM card except Orange's own. By unlocking the Orange handset, you can effectively pop in the T-Mobile SIM card in place and continue using the handset as if it was "bought from T-Mobile" instead.
A unlock code would be required (There are plenty of websites that can do this for you for a fee), some operators will charge you for the unlock code - again this is dependent on whether you're on contract or pay-as-you-go - check the terms and conditions of that first. For example, from my dealings on modaco's forum, Orange will not give you the unlock code until after a time period of usage on contract, think it was 3 months, again check!
Once a unlock code is handed over to you, this is taken from the handset's IMEI number and the designated SIM card that you wish to use, which a 6 or 8 digit number is generated, that, going by the previous example, a T-Mobile SIM card gets popped in, the Orange's handset will prompt for the network unlock code, in which the 6 or 8 digit number is entered to unlock it thus freeing the handset from the network.
One thing though, this happens with Samsung, flashing unofficially using the likes of Odin or Multiloader, it could mess up the handset, because there's a checking in place to ensure that if the handset is locked, the flashing will fail. KIES is one! Now, this was going back about 18 months ago, so I do not know if that applies now.
If you are thinking of flashing a custom ROM, double check to ensure that the handset can indeed be flashed with the network lock in place.