Programmatic SEO is building many search-targeted pages from one template and a dataset, such as a page for every city, every integration or every "X vs Y" pair, instead of writing each by hand.
It works when the data behind each page is real and different, so every page answers its own search properly. Directories, comparison sites and converters are built this way.
It fails when the pages are the same text with one word swapped. Search engines treat that as scaled, low-value content and can stop indexing the whole site.
Why it matters for a launch
For a maker it is worth considering only once you have data worth a page each. Until then, a handful of pages written carefully will do more than a thousand generated ones.
Read more: Why a new website isn't on Google yet