The copy command creates a copy of its argument, which is typically a list of some type. If B is a list and A:=B, then A and B point to the same list, and so changing one will change the other. But if A:=copy(B), then A and B will point to different lists with the same values, and so can be changed individually.
| B:=[[4,5],[2,6]]:; A:=B:; C:=copy(B):; A,B,C |
|
| B[1]=<[0,0]:; A,B,C |
|