Very simple - One Megabyte of memory on an expansion card.
The memory could not be accessed from Basic but could be paged in at machine code level and was most useful to use as a RAM Disc with MasterDOS.
Most users with the One Meg also had a SamBus or a Two-up/Three-up interface expansion unit.
On the circuit board there is a jumper connection (bottom left above the capacitor) to allow a one from four position selection (which is factory set to position four). This enables up to four 1 Mb External Interfaces to be connected together giving a possible total memory capability of 4.5 Megabytes of RAM (4,718,592 bytes)
Some demos from Dave Tonks (Walker Demo) and Guy Middleton (Top Gun Demo) also made use of the extra memory.
A few products utilised the extra memory such as E-Copy by Chris White, SAMPaint and version 2.0 of Pro-DOS.
Due to a lack of memory contention, code running in External RAM can run faster as shown in the Speccy.pl Party 2019.1 Invitation.
More recently productions such as Rick Dangerous have used it to implement additional features, in this case a 'save state' system.