It's a built in single smart card reader, quite common to find in E2 and many other third party receivers for that matter. It's for use with subscription viewing cards but as such on it's own won't do a great deal of anything, it can be used in conjunction with various 'softcams' which can be installed in Enigma2 to enable you to make use of it.
This unfortunately though doesn't mean that every viewing card in the world can just be plugged in and work, that's not going to happen I'm afraid but there are many viewing cards that can stil be made to work and likewise many that won't, this part as such though is not unique to the receiver hardware or smart card reader, they essentially are what they are, it's the softcam part which is the thing that can connect to two and the type (providers) card you want to use which will ultimately dictate what can and can't be used.