Help with understanding: xxxx of the xxxx rows in the data stack have missing values, and will be omitted in the blending process.

When I am calling my ensemble stacked model

I get the warning:
2506 of the 7102 rows in the data stack have missing values, and will be omitted in the blending process.

I found the warning message documented here tidymodels/stacks source: R/blend_predictions.R but what does this really mean?

"{nrow(data_stack) - nrow(dat)} of the {nrow(data_stack)} rows in the ",
"data stack have missing values, and will be omitted in the blending process.