A "board" is another name for an internet forum, originating (as far as I know) in the long-ago days of online "bulletin board systems," which were commonly referred to as boards. It's not a term that gets much usage these days, and it will fade a little more later this month when Riot closes the League of Legends Boards.
"Starting March 9, 2020, we'll be closing down the League of Legends Boards," Riot said in the announcement. "We know some of you use them regularly, so we didn’t arrive at this decision lightly. However, after looking at how many of you were using Boards, it became clear that the popularity of the platform had decreased significantly. Both players and Rioters have migrated to other community discussion platforms such as Reddit and Discord, leaving Boards underpopulated."
"We want to give a huge shout-out to our volunteer moderators! Over time we saw a volunteer moderation program emerge for those who were passionate enough to support players in their local regions. They put a huge amount of work into hosting community events such as cosplay and fan-art contests. They were ever-present on Boards to talk with players and assist them. To the volunteers who have spent the past decade moderating and helping the community on Boards, we cannot thank you enough!"
In order to give users a chance to archive anything they may want to keep, the Boards will be made read-only on March 9, but will remain accessible. One week later, however, they'll be taken offline completely, and anything not saved will be lost.
The reaction to the coming closure on the boards themselves seems muted, which is probably an apt justification for Riot's decision to pull the plug. Even so, and even though I'm not an LoL player, I always find it sad when traditional forums fall by the wayside: I'm still a little sore about BioWare closing its own forums back in 2016.
But you can't fight progress, I guess, and fortunately there are lots of other places where League of Legends fans can keep up with community happenings in a post-Boards world. Our very own PC Gamer Forums are a friendly spot for discussion, too.