What the excerpt field actually feeds
The excerpt field on a Squarespace blog post feeds three surfaces. The blog index page (the post listing renders the excerpt between the title and the 'read more' link). The RSS feed (the excerpt becomes the item description). Social-card previews when no separate Open Graph description is set (Facebook, LinkedIn, link previews in iMessage). What the excerpt does not feed is the meta description — the snippet Google shows under the post title in search results. That field lives on the post's SEO panel as a completely separate input.
The meta description gotcha
The pattern that catches owners: write a thoughtful excerpt, see it render on the blog index page, assume the work is done. The post ships without a meta description. Google fabricates one from the post body — usually picking the opening paragraph, which on a Squarespace post is often the first sentence of the lead. The fabricated snippet is not necessarily wrong, but it is usually not optimal for click-through from search results because it was not written with the search-result context in mind.
Writing excerpt and meta description separately
The excerpt should read as an editorial teaser — one to two sentences that introduce the post for someone already on the blog index page. The meta description should read as a search-result snippet — 150-160 characters with the primary keyword included once, optimised to drive a click from search results. The two will often share themes and even some phrasing, but they should not be identical. The excerpt can be conversational; the meta description needs the keyword.