I'm new to the forum so I hope this isn't, like, a common/annoying question, but..
I haven't played the game in a few years. Last I played it frequently [/i]they'd just introduced the rabbit, and the last time I played it at all[/i] was just after they introduced cement. Don't know what's happened since then, but I heard something about marine mobs and that makes me think that I can't just jump right back in like nothing's changed.
Personally, I would recommend just jumping back in, and discovering the changes yourself. There are certainly people on the forum that would relish the chance of having that feeling of freshly discovering new aspects of Minecraft -- the feeling of exploring the unknown, rather than seeing the new stuff listed in a change log.
You can always come back to the forum, or visit the Minecraft subreddit or wiki if you need more information about something you find.
There haven't been that many updates since you last played if you played with concrete, which was added in 1.12, while the current update is 1.13 (one major update later) and 1.14 will soon be released (there is about one major update per year; minor updates, like 1.13.2, are mostly bugfixes, with some being very minor feature updates (e.g. 1.6.2 added baby zombies and 1.7.4 added chicken jockeys). So you'll pretty much have the same experience as somebody updating from 1.12 to 1.13.
Just New it? I tended to only read the Wiki in my first few Years waiting-through the night.
Now Night's a real thing! Phantoms come after an Hour without a Bed!!
Believe me, there're better ways to spend time with MC than reading - the Wiki - or here *ignores self* . If nothing else, Chat it out in M(ulti)P(layer), watch YouTube if (actually) Hungry, and try holding Left-Click (took me Hours to figure out how to Break and Place, Blocks).
Soon 1.14 will be released, so if you plan to update to that it’s worth reading up about the upcoming pillagers beforehand. Other than I’m sure that you’ll have a good experience just diving in where you left off.