a = np.array([1, 2, 3, 4, 5, 6, 7, 8])

c = a[2:]

d = a[:-2]


as a reminder.
The first element is: a[0]<-- index=0
a[0]=1
The second element is: a[1]<--  index=1
a[1]=2
The third element is: a[2]<--  index=2
a[2]=3
The fourth element is: a[3]<--  index=3
a[3]=4
The fifth element is: a[4]<--  index=4
a[4]=5
now

c = a[2:]

will create an array c that will not include the first two elements. the output is

[3, 4, 5, 6, 7, 8]


on the other hand.

d = a[:-2]

will create an array d that will not include the last two elements. the output is

[1, 2, 3, 4, 5, 6]