I remember something like the “event horizon” ?
Something about a limit beyond which we can’t see back into the early universe..
Ring any bells?
The term event horizon refers to the boundary region around a black hole within which gravity is so strong even light cannot escape. Events within that horizon are thus barred from interacting with events outside it, as no signal can travel outward successfully (up to a quibble about quantum effects right at the boundary).
What you are thinking of is the epoch when the universe became transparent. Originally theory says it was a plasma - like a fireball, all particles "ionized" or in other words stripped of their electrons which fly around free themselves - which would not transmit light, since any photon would collide with something (usually one of the electrons) before getting far, and be scattered again.
That is as far back as one could theoretically "see" in time with "old light", because light earlier than that couldn't travel. Once the universe cools enough that electrons are captured by atoms, it becomes transparent to light, and light emitted from that point on might reach us now, if it started far enough away.
The cosmic microwave background radiation - a diffuse, nearly uniform microwave "hum" seen in all directions, at an absolute temperature of 3 degrees kelvin worth of energy, is thought to be the afterglow of that epoch, or the "first light" that could get to us now. Not that there wasn't any earlier, but it would have been totally scrambled by multiple absorptions and re-emissions (aka collisions or scatterings) in the plasma etc.
This is usually called the epoch of transparency or something similar, but not "event horizon".
700 million years is long after it is thought to have occurred, though.