Toners are chemical treatments that may:
(1) help to make the photosensitive silver particles more stable, or
(2) impart a color tone to the print, or
Historically, some of the most prevalent are so called “sepia” toners, usually sulfides that lend a yellow-brownish tone to the photo. Partial, or "split" toning with sepia tends to impart a sort of glow, with the lighter tones gaining a hint of warm color while the darks remain black.
Selenium is a prevalent toner today, which many photographers use in a watered down version in an attempt to avoid a color change. When allowed to do its work completely, it lends a sort of plum color tone to a B&W print.
Toning with gold chloride is expensive but tends to produce cooler, bluer tones, unless combined with sepia toners, in which case some pink surprises can happen.
All three greatly increase the life span of a black and white (monochrome) photograph. Blue toning and copper toning (a reddish color) do not add to the archival status, but do change the color, sometimes drastically. Many other colors and combinations are possible.