In this context, a pantheon is the group of gods connected through a religion or a group of people. How pantheons work can depend on how you have religion operating in your world. As D&D is set in a fictional world, there are many fictional pantheons, but D&D also offers historical pantheons as potential religions in your campaign setting, such as the Greek and Egyptian pantheons.
Name, title | Alignment | Domain(s) | Symbol
A campaign setting that has been in various editions of D&D, the Forgotten Realms typically refers to the continent of Faerûn, though worship of Forgotten Realms deities can exist throughout the world. You don't need Faerûn in your campaign to use Forgotten Realms gods.
Greyhawk is a campaign setting for D&D. Consisting of various settlements and locations, the world of Greyhawk offers many options for a campaign. Your campaign does not have to take place in Greyhawk to use Greyhawk deities.
Dragonlance is a fantasy setting that can be used in a D&D campaign. Widely popular as a literary setting for fantasy stories, the Dragonlance pantheon can be found on the continent of Ansalon. The Dragonlance pantheon gods are typically categorized as Good, Neutral, or Evil, though they all exist under the umbrella of the Dragonlance pantheon. You do not need to use Ansalon or any other Dragonlance setting to use Dragonlance deities.
Individual races can have strong ties to particular gods. Whether these deities are responsible for creating the race or had a hand in shaping their culture, nonhuman deities typically have a more wide-spread influence in cities created by individual races--the Dwarven pantheon may be more widely worshipped in a Dwarven city, etc.
Under construction! Fun fact about the Fantasy-Historical pantheons: though they are based on pantheons in our world, they have histories and stories that are specific to the D&D world. Sorting between what is historically accurate (for us) vs. how D&D frames these gods is a real headache! So, think of this section as a "coming soon!"
Eberron is a campaign setting for D&D, a land wrought with magical secrets and the slow recovery from a horrific war that saw new monstrosities and weapons created. Eberron has two prominent pantheons, one on the side of good and one on the side of evil. In addition, there are a few miscellaneous faiths that also exist in Eberron. The gods of Eberron do not abide by typical alignment rules, and as such, they do not have denoted alignments. You do not need to have a campaign take place in Eberron to use the Eberron gods.
Based on a setting for Magic: the Gathering, Theros is a campaign setting for D&D inspired by Greek mythology. Theros is a world where the gods are active and more than willing to use mortals to advance their own desires. In a world where adventurers can be destined for glory, the Theros pantheon offers gods that may take an interest in your character. You do not need to use the Theros campaign setting to use the Theros gods.