Anchor text is the visible, clickable text of a link. In "try our invoicing app", the words "invoicing app" could be the anchor text.
Search engines use anchor text as a clue to what the linked page is about.
Natural links use a mix: your brand name, the bare URL and descriptive words. A sudden pile of identical keyword-rich anchors can look manipulative.
Why it matters for a launch
When you get to choose, as on your own site or a guest post, descriptive anchor text helps the linked page rank for what it's about.