var arrNum [5]int // this is an array
var sliceNum []int // this is a slice

fmt.Println(arrNum) // [5]int{0,0,0,0,0} array have initial value of zero value
fmt.Println(sliceNum) // []int(nil) slice have initial value of nil value

sliceNum == nil // true

arrNum[0] = 1 // ok
sliceNum[0] = 1 // compile time error because you cannot assign values to nil

var arrNum2 [3]int
arrNum = arrNum2 // compile time error because length is part of array type

sliceNum := []int{0,0}
sliceNum2 := []int{1,2,3}
sliceNum = sliceNum2 // ok, you can assign slice of diffrent length to a slice