The tensor to scalar ratio is a cosmology observable the looks at patterns in the distributions of what we see in the sky collectively with telescopes to constrain the kinds of gravitational waves that left an imprint on the distribution of stuff in the early Universe. It is used primarily to constrain the hundreds of possible theories of cosmological inflation - a momentary period of unfathomably rapid expansion of the Universe almost immediately after the Big Bang which would help smooth out the universe.
A new paper analyzes the Planck satellite data and other observations to establish a new boundary on the scalar to tensor ratio to r < 0.044 (95% CL), the tightest limit on r to date. The best fit value that is theoretically possible is r = 0 and there is (crudely speaking) a 68% chance given the data that the true value is r < 0.022.
This narrows the box shown in this illustration of July 2018 results (with my annotations) from Planck of r < 0.064, by about a third along the Y-axis.