2-pole means that the device plug is not earthed and that it normally has two pins which transmit electricity.
Originally, all electrical devices were fitted with 2-pole plugs, which mean that the devices were not earthed and that all mains sockets were constructed for 2-pole plugs. With the predominance of more and more powerful devices, the risk of an electric shock increased. This problem was solved with the installation of an earth connection (mostly in the form of a 3rd pin). This is why earthed plug systems are known as 3-pole systems although this is not quite technically correct.
On top of this, 2-pole plugs are not constructed to continuously withstand high currents. If more than 2.5 amperes, in other words more than 500 watts, are transferred through a 2-pole plug, it is possible that damage caused by smouldering or burning can occur under certain conditions since the plug pins are often not thick enough to continuously resist the heat caused by the higher power.
In the case of less powerful devices (normally up to around 500 watts), 2-pole connections are still the norm since these can be produced more cheaply. You can find a device overview here.
In most countries in the world, the 2-pole mains sockets have been replaced with earthed sockets which can accept both 2-pole and 3-pole plugs.