Here is a quick explanation using comments. Expect a much more detailed explanation from Jeff, but hopefully this helps get you closer to understanding what it does and how. Also, I did make a few minor changes.

One question up front, is this supposed to generate 6 years of values?

WITH

e1(n) AS (SELECT * FROM (VALUES(1),(1),(1),(1),(1),(1),(1),(1),(1),(1))x(n)), -- generate a dynamic table with 10 rows

e2(n) AS (SELECT a.n FROM e1 a, e1 x), -- cross join e1 with itself to generate a dynamic table with 100 rows

e4(n) AS (SELECT a.n FROM e2 a, e2 x), -- cross join e2 with itself to generate a dynamic table with 10,000 rows

e8(n) AS (SELECT a.n FROM e4 a, e4 x), -- cross join e4 with itself to generate a dynamic table with 100,000,000 rows

cteTally(n) AS (SELECT TOP 6307204 ROW_NUMBER() OVER(ORDER BY (SELECT NULL)) - 1 FROM e8),

-- select the first 6,307,204 rows of e8 and number them sequentially starting with 0 (the -1) to 6307203 (6307204 - 1)

Test(n) AS (SELECT DATEADD( minute, n, DATEADD( YEAR, -2, GETDATE())) FROM cteTally)

-- Starting with the current date and time minus 2 years, add n minutes to the datetime

select

DATEADD(mi,datediff(mi,0,n),0)as cte_start_date -- drop (or zero) the seconds from the date time value

INTO

Sandbox.dbo.Time

FROM

Test

WHERE

n <= DATEADD( YEAR, 4, GETDATE()) -- don't think this works as expected, see question above.