A spoiler reveals a part of the plot or ending, for example what finally happened to that One Ring. If you have not yet read The Lord of the Rings (or the concerned work), then you might not want to read a spoiler.
This particular wiki sometimes warns readers of approaching spoilers. Many already know the plot and ending of The Lord of the Rings and Tolkien's other works, so the spoiler warnings might be unnecessary; many articles do not have them. However, some documents open with a few spoiler-free paragraphs, then give a warning before providing any spoilers. This is probably a legacy of adapting some content from Wikipedia, a general encyclopedia that attracts more readers who are less familiar with Tolkien.