Two things wear this name. A true secondary is insiders or early backers selling shares they already own: no new shares are created, so there is no dilution, but the float grows and a large holder is exiting. A primary follow-on is the company itself issuing new shares, which does dilute. Read the prospectus line that says who receives the proceeds.
Both are usually priced at a discount to the last close to clear the block, and both tend to cap the stock near the offer price for a while because the underwriters are holding inventory.
Example: a $30 stock does a 10M share secondary priced at $28.20, a 6% discount. If all 10M shares come from a founder's stake, share count is unchanged and the company receives nothing; the $282M goes to the founder.
Related: follow-on-offering, dilution, offering-discount, underwriter, float