It's an addressing mode called "Indirect addressing with index and displacement"
x = Index, an 16-bit immediate value
An = An Address Register (A0,A1,A2,A3,A5,A5,A6,A7)
Dn = An Data Register (D0,D1,D2,D3,D4,D5,D6,D7)
scale = an 4-bit immediate value which can be 1, 2, 4, or 8. Can use .b,.w,.l,.s too.
Which points to content inside resultant address obtained by the sum of values:
; put cursor at start of RAM and fill with some bytes
; put cursor at start of RAM again
; move a content using the indirect address mode
; with index and displacement to offset 0x40 of the RAM
; After this instruction the value in offset 0x40 of the RAM
; will be the same as 0x6 of the RAM, which is 7080
You can use a m68k simulator to learn and understand all addressing modes or opcodes
/ and https://web.archive.org/web/20200219135847/http://www.easy68k.com/index.html