The last time I played Yugioh with any consistency was back in 2015. If I wanted to get back into it, how do I go about learning all the new rules/mechanics and all that? It’s intimidating lol
If you're from 2015 then the only thing other than newer cards to learn about would be Link Summoning and the update to Pendulum Summoning. Link Summoning is dummy simple, too. I'll run it down for you:
This is a Link Monster. They only have ATK and cannot be in Defense Position. You summon them like you do Synchro monsters. If you have the materials on field, you send them to the grave and slap the monster down on the field. The "LINK - 3" is the Link Rating, which is how many materials you need to summon it. This is a Link 3, meaning you need 3 materials.
"So why does it only say 2+ Effect Monsters?"
Link Monsters themselves can be used as either 1 material or an amount of materials equal to their Link Rating. So this monster, Decode Talker, can be used as either 1 or 3 materials for another Link Summon.
"So what are the arrows?"
The Link Arrows are where you can summon more Link Monsters. Yugioh has a rule where if you're special summoning Link monsters from the Extra Deck they either have to go to the Extra Monster Zone (one of the two spaces in the centre, of which you are allowed to use one of them under most circumstances) or to one of your Monster Zones a Link monster points to. So if this Decode Talker is in the Extra Monster Zone, you can summon another Link Monster to one of the lower two Monster Zones it point to:
(s/o to u/
DeKernelm on reddit)
This also came with a rule stating that Pendulum Monsters that are face-up in your Extra Deck have the Link Summon restriction on them as well (only to the EMZ or zones Link Monsters point to).
and before you ask, no you cannot use your opponent's zone that it points to...but they can.
Oh, and they also got rid of the pendulum zones and moved them to the 1st and 5th Spell/Trap Zones which you can also see above.
Any other questions? I swear this might be a decent amount of text but in practice it's very simple to understand. There are some other mechanics too, like monsters being Linked (one link monster pointing to another), Co-Linked (Two Link monsters pointing to each other) and Extra Linking (having a chain of Link Monsters going from one EMZ to the other, the only way you're allowed to use both EMZs). You rarely see Extra Linking though unless your opponent is playing a specific deck for it, like Cyberse.