Sim slot is a place in a phone that are design for inserting SIM card. Phone come with different numbers of som slots.
Some have one, others have two and some have more than two slots for inserting two different SIM cards. Phone with two or more sim slots enables the user to avail mobile connectivity services from more than one service provider.
One can even insert SIMs of several different in the slots that are available in the phone.
This enables the user to carry one phone, rather than juggling between two handsets. Today, you can purchase a truly handy cell phone that has multiple SIM card slots.
You can switch between the two cards by pressing a simple button which is conveniently located on the body of the phone.
These new models enable you to keep all your cards active at any given point of time or switch off one or two as per your convenience. When you shop for your multi SIM or double SIM phone, look for the terms DSDS i.e. dual SIM dual standby or MSMS i.e. multi SIM multi standby.
A demo video form youtube showing you how to simply a way to open the iPhone SIM tray if you have a iPhone.