If you want to edit a file that is checked in to source control, you must go to source control system and "checkout" the file. All you have to do is, right click on appropriate file in the source control explorer and select "Checkout File". The source control system will remember the folder structure you specified and copy the latest version of the file to the appropriate folder.
When you checkout a file from Source Control, it will copy the latest version of that file into your computer. This will overwrite your copy of that file. So, if you have made any changes to the file in your computer without checking out, those changes will be lost during this process. It is very important that you should not change any file before checking out the file from source control.
When a person checkout a file, the source control system will mark that file as 'locked'. This means no other user can checkout the same file until you checkin that file. This will ensure that the same file will not be edited by different people at the same time.
How to cheat source control ?
There is no way a source control system can stop you from editing a file that is in your computer. Only thing the source control can do is, make the file readonly to remind you to checked out the file.
You can always go to the windows explorer and change the 'Readonly' attribute of the file. This will allow you to edit the file!
But not that by doing this, you are cheating the source control. You cannot checkin the file you changed (since you have not checked out the file). If you somehow manage to checkin the file to source control, you may end up messing up the file. Some other members may have changed the same file and you may overwrite his changes by checking in your version of the file. (If you had checked out the file before you changed, then nobody else can checkout that file and change anything which will ensure that you will not lose other's changes on the same file). It is easy for someone to look at the 'history' of the file in source control and find who did what and when. So never attempt to cheat source control systems by changing the 'Readonly' attribute.