Back when mainstream American comic books began, early in the 20th century, there was only one narrative and only one fictional universe. This lasted for some decades, but at one point, publishers began feeding us with so-called alternative stories, which gave birth to the concept of the Multiverse.
The Multiverse is a concept of a larger fictional universe within which there exist multiple alternative realities, often called “Earth”, with one primary, canon narrative setting; in Marvel’s case, it is Earth-616. This allowed for more diverse storytelling and enabled a constant expansion of the fictional narratives.
The concept of the Multiverse might seem difficult to understand, but that is quite misleading. We are going to explain everything step-by-step and before you’re done, you’re going to be an expert in the field. We are also going to explain how the MCU universe ties into the whole concept.
What is Marvel’s Multiverse?
Within the narrative universe created by Marvel Comics, most tales take place within the large fictional Marvel Universe, which, in turn, is part of a larger fictional Multiverse. Beginning with the early issues of Captain Britain, the main continuity (the so-called “Prime Earth”) in which most of the Marvel stories take place was labeled as Earth-616, and thus, the Multiverse was established as being protected by Merlyn.
This was back in 1976, although the concept was vaguely introduced as early as The Avengers #85 (February 1971). 
The concept worked so that each universe has an iteration of Captain Britain appointed to protect their iteration of the British Isles. These protectors are collectively known as the Captain Britain Corps and they actually represent the initial evolution stage of the Multiverse.
Gaining popularity, this numerical notation of the individual Earths was continued in the Excalibur series and other titles before pouring over to the core titles.
Each alternative universe within the Multiverse in Marvel also had its own Sorcerer Supreme at almost every moment in their history, always appointed by the mystical trinity of Vishanti in order to defend the world against threats of primarily magical nature from within and beyond and always wielding the Eye of Agamotto, the Sorcerer Supreme’s most powerful weapon.
Later, many writers would use and reshape the Multiverse in different titles, with some notable being Exiles, X-Men, and the Ultimate Fantastic Four; the Ultimate Marvel Universe is especially noteworthy in this context.
New universes would also emerge from stories involving time-traveling characters like Rachel Summers, Cable, and Bishop, as their actions made their origin times alternate timelines.
How many universes are there within Marvel?
The Marvel Multiverse opens the possibility for an infinite number of alternative Earths, as it is not, unlike DC’s Multiverse, limited in any way. The exact number of alternative Earths is unknown, but here is a list of some of the more prominent ones:
How could the MCU fit into Marvel’s Multiverse?
The integration of the MCU in the Multiverse was a question that arose back when Iron Man first appeared on the screen. And while a lot of people thought it would be impossible, the MCU has – in fact – already been integrated into the Multiverse.
Namely, the MCU’s reality has retroactively been labeled as Earth-199999 and has been integrated into the larger Multiverse as an alternative Earth to the primary Earth-616. This means that the MCU is, officially, part of the larger Multiversal canon.