There are three schools of thought surrounding tank play. The first is probably the truest to its role. The second is a requirement in almost every composition. The third is what dps players queue when they get bored.
To understand this role, you need to understand space. This is something that lower ranks just don't understand: it is NOT your dps's job to make space so you can walk through a choke. Let's take rein as an example. You are the big alfa tank. You need to push your way through a choke so that your dps's sightlines open up and your support can take better positions. The longer you, as rein, sit in a choke, the more resources your team is going to have to pump into you. You need to learn when to take and give space. Times to take space is when your dps and supports need sightlines onto the enemy team. The more constricted the area is that you're in, the less your team can do. SPACE.
The primary characters of this role are Zarya, d.VA, and Sigma. You're tank partner with dictate what off tank you pick. What you do as a character is andd to the team damage, and enable your other tank. Each off tank does it a bit differently. Zarya is pretty much a dps character with bubbles. Use them to enable your Tank while you kill everything. d.Va is really good at taking off angles, peeling, and contesting snipers. Sigma is the best off angle character in the game. His damage is massive but you lose out on some utility. He's mainly played in poke.
This section will only be covering Hog and Wrecking ball. I won't be going in-depth on the characters; that's what the hero guides are for. Just briefly touching on the characters, Hog gets most of his value out of a very selfish playstyle. Harrassing backlines, looking for hook widows, and shredding shields will get you the most value out of the character. Wrecking ball is similar but his skill and value ceilings are way higher. If you're smart with ball, you'll be setting up team for free kills as Ball is all about displacement and picking off stragglers. Both can be good in metas but you have to understand how they work in the composition.