They are talking about cell lines taken from aborted foetuses, used during research into vaccines. No different from cells or for that matter, cadavers used in medical research for many reasons.
The foetuses were not aborted for the sole purpose of using the cell lines, and there is no evidence that these were electively aborted foetuses, or naturally aborted foetuses.
This is one of the oldest and most debunked, anti-vaccine ploy.