TTRPG = Table Top Role-Playing Games.
If you have never heard of them or have never tried one, the most popular one, though not my favorite from a rules or story perspective is a game called Dungeons and Dragons. Now in its fifth edition, moving into its next edition soon, D&D has been running for decades, and has the easiest time finding a table of people to play with. Though if you are willing to play digitally, roll20, and other similar such platforms enable you to find games filled with other players for other systems like Cthulhu, Pathfinder or GURPs, much more easily than ever before.
But back to the topic at hand. Why am I saying that you should play them at all?
Two reasons:
1) If you read and write fantasy, it will help you learn how to problem solve inside of a set of rules, just like authors do when they write themselves into corners. This useful to a degree. Item 2 is WAY more important.
2) You get to see what the players find to be important. The people who play these kinds of fantasy and science fiction role playing games are often the people who read books in the same genre. Because of this you can get a very up-close view of what is important to the readers in your genre? What kind of adventures do THEY want to go on?
I grant this is a limited scope, and will introduce you to 5-10 people on the outside for a single sitting, but it is still insightful to see what things people think about, how they perceive wizards, thieves, fantasy creatures and why.
Is it everything the fantasy and science fiction writer needs to know about writing? Not even close. It is however a very useful took to begin to glean some things from your fellow players, and the adventure just might lead you to new storylines.
Give a game a go, and always remember. Writers Write. Try our series on dialogue, Conflicts, and how not to annoy your reader.