It is the bigger copper line running from the evaporator coil in your furnace to the condenser coil outside. It is bigger cause it carries vapor. It carries hot refrigerant vapor from the evaporator coil and goes into the compressor which then forces the vapor into the outside condenser coil where the refrigerant cools down and becomes a cool liquid for the return trip to the evaporator coil in the furnace/air handler. The return line is smaller cause it carries a compressed liquid. The fan at the condenser coil pulls air through the coil to cool the refrigerant and to cool the compressor. As the furnace air handler/fan blows air through the evaporator coil a heat exchange takes place where the heat from the air transfers into the refigerant and the coolness of the refrigerant transfer into to air. The evaporator coil in your furnace plenum is a triangular shape series of copper coils with fins like the outside unit.
Too answer your question now, the suction line is hot and needs insulation so it doesn't sweat and drip all over everything underneath it. It is basically the 3/4" foam pipe insualtion that you can buy for a couple bucks a 5' stick. Double it if you want.