Marvel's symbiotes are parasitic alien lifeforms, the Klyntar, that bond with others across the galaxy, generally the strongest hosts they can find at any given moment. The most famous of these is Venom, which was introduced by bonding to Spider-Man during the events of the original Secret Wars. Since then, it's been bonded to multiple people, most notably with Eddie Brock.
However, some variants of the symbiote -- as well as the core Marvel Universe incarnation of Venom itself -- have found themselves attached to perhaps Marvel's mightiest mortal: Bruce Banner, aka the Hulk. And with recent events in the Marvel Cinematic Universe, that could become a reality in that world too.
The Venom Symbiote is able to possess others and adapt the skill sets and knowledge of past hosts to their current one. This makes any incarnation of Venom a dangerous creature, especially after it can duplicate the strengths of figures like Spider-Man. The only consistent weaknesses that the creature has ever shown are to fire and intense sonic waves, which negatively affect the creature. The Hulk, meanwhile, is a rage-induced transformation in Bruce Banner that drastically increases his strength and endurance. The Hulk is among the single strongest beings in the Marvel Universe, capable of going toe-to-toe with the likes of Thor and Thanos. Amplified by the symbiote's own enhanced abilities, a Venom-Hulk is an incredibly powerful force.
On Earth-616, the Venom symbiote has encountered and briefly bonded with two versions of Hulk: during his tenure with the symbiote, Flash Thompson ended up lending the symbiote to Thunderbolt Ross. Meanwhile, in an effort to protect Brock's son Dylan during the events of Absolute Carnage by Donny Cates and Ryan Stegman, the symbiote abandoned Eddie to merge with Bruce Banner. Coming during the period where Bruce was contending with the Devil Hulk (a particularly malicious variant of the jade giant that was the center of the recently concluded Immortal Hulk series), the fully Venomized-Hulk battled a Carnage whose powers had been amplified by his connection to the symbiote God, Knull. Despite the power boost, Carnage targeted his brain specifically, weakening Hulk enough to rip Venom from him.
In other worlds across the multiverse, variants of Venom got the chance to more permanently bond with the Hulk. On Earth-1089 -- as featured in What If...? #4 by Danny Fingeroth and Mark Bagley, the Venom Symbiote was able to more successfully bond with Spider-Man. Taking control of him, the symbiote fed off his life force until it encountered the Hulk mid-battle with Doctor Strange and the Avengers. Sensing his power, the symbiote abandoned Peter for the Hulk -- gaining control of the Hulk while leaving a withered and aged Spider-Man on the verge of death. The Avengers later encountered the Venom-Hulk after it had finished absorbing the gamma energy from the Hulk, and was able to trick Thor into allowing it to possess him until Black Bolt arrived to kill the creature once and for all.
In their current film incarnations, it seemed unlikely Venom and Hulk would even encounter each other, let alone have the symbiote hop to Banner. The Hulk (Mark Ruffalo) is a resident of the primary MCU timeline, while Venom has been operating removed from the MCU. But the post-credits sequence from Venom: Let There Be Carnage featured Eddie Brock (Tom Hardy) saw reality shift around them and seemingly deposit them from a small room in their reality to the MCU -- where Venom sees the MCU incarnation of Spider-Man (Tom Holland) on television, his identity exposed. Hopping realities would make sense given the dimensional shifting events teased in the trailer for Spider-Man: No Road Home -- giving an easy reason why Venom would be in this reality.
This is a major shift for Venom, and opens the door for the Lethal Protector to encounter plenty of characters -- including potentially the Hulk. While it's not as likely as a confrontation (or potentially even a team-up) between Spider-Man and Venom, there's always the chance that Venom could sense the true power lurking in the Hulk and could decide to try and take it for its own.