In Aquinas's first proof of God's existence, what is the difference between accidentally and essentially ordered causal series?

I'm reading «Teología Natural» by Ángel Luis González. In the book, it is stated that St. Thomas Aquinas' viae would allow an indefinite series of causes. He distiguishes between two causes. The first is the "cause essentially subordinated in the present", and the "cause accidentally subordinated in the past". Only the latter can come from a process in infinitum. He even quotes St.Thomas: *S.Th.*, I, q.46, a.2, ad 7. I'm having a some difficulty in understanding what these two types of causes are. Any help would be appreciated