minetest-flow/doc/tooltips.md

Tooltips

You can add tooltips to elements using the tooltip field:

gui.Image{
    w = 2, h = 2,
    texture_name = "air.png",
    tooltip = "Air",
}

There is also a gui.Tooltip element which lets you change the background colour of the tooltip. As with gui.Style, it is invisible and won't affect padding.