background-repeat modifies how backgrounds will repeat. You can modify the repeat of both the x and y axis.
A few notable values are:
Attempts to place as many repeated backgrounds as possible without stretching or cropping any backgrounds. Any empty space is added between backgrounds.
Fairly hard to describe so instead I’ll quote from the MDN page on background-repeat.
As the allowed space increases in size, the repeated images will stretch (leaving no gaps) until there is room (space left >= half of the image width) for another one to be added. When the next image is added, all of the current ones compress to allow room. Example: An image with an original width of 260px, repeated three times, might stretch until each repetition is 300px wide, and then another image will be added. They will then compress to 225px.