app/themes/hugo-theme-stack/layouts/partials/data/description.html

<!-- Use site subtitle by default -->
{{ $description := .Site.Params.sidebar.subtitle }}

<!-- Seprate description exists -->
{{ if .Site.Params.description }}
    {{ $description = .Site.Params.description }}
{{ end }}

{{ if .Description }}
    <!-- Page description exists -->
    {{ $description = .Description }}
{{ else if .IsPage }}
    <!-- Use page summary -->
    {{ $description = .Summary }}
{{ end }}

{{ return (replaceRE "\n" " " $description | plainify) }}