You can detach the keyboard of your device and use the screen as a tablet. Detaching the keyboard makes the device lighter and gives you the possibility to switch easily between laptop-mode and tablet-mode.
Retina display refers to a high-resolution screen technology developed by Apple. A Retina display has a high pixel density, with more than 300PPI, which makes it impossible to spot pixels from a normal viewing distance (10-12” from the eyes).
More memory slots (also known as DIMM slots) allow you to add more RAM to your computer. It is also useful when upgrading, as you can add RAM to an empty slot instead of replacing an existing memory module.
The device has a standard memory slot (such as an SD or micro SD card slot) so that you can either extend the internal storage with affordable memory modules or you can retrieve data, such as photographs, easily from a memory card.