What does it mean for a slice to have a default value of zero, and what is its essence?

In Go, a slice is a structure that contains a pointer to an array, a length, and a capacity. The default value for a slice is nil, but if a slice is initialized as empty, for example var s []int, then its value is nil, and its length and capacity are zero.

When it is said that the default value for a slice is zero, it usually means that the length and capacity of the slice are zero, and it does not point to any array.

The point is that such a slice does not contain elements and does not occupy memory for data, but it can be safely used, for example, to add elements via append.

Example:

var s []int // s == nil, len(s) == 0, cap(s) == 0
s = append(s, 1) // now s contains one element

Thus, the default value of a slice is an empty slice with length 0, which is convenient for working without additional initialization.