What does ">_" mean in run command?

I found this grammer in workflow files:

      - name: Build wheels and source tarball
        run: >-
          poetry build

what does “>_” mean?

That’s part of YAML syntax for multiline strings. The > says to fold lines within the following block (replace newlines with spaces), the - removes newlines at the end of the string.

See also: https://yaml-multiline.info/