A fixed grid in Figma is a layout grid where you set specific widths for columns or heights for rows, along with the number of columns or rows. This is especially useful for creating detailed designs where precise alignment of elements is crucial.

To apply a fixed grid:

1. Select your frame, then click the + next to Layout guide in the Design panel.
2. Choose between Column or Rows by clicking on the Layout guide settings icon.
3. Set your desired width or height and the number of columns or rows using the settings window that pops up.
4. You can fix rows to the top, bottom, or center of the frame, and columns to the left, right, or center under the Type dropdown.

This provides a structured, consistent layout that enhances usability and aesthetics in complex interfaces.