Dilution is arithmetic, not opinion. If the share count rises and your holding does not, your slice of the company shrinks. It happens through a secondary-offering, an at-the-market-offering, stock-based-compensation, warrant exercise, or conversion of a convertible-bond.
Dilution is not automatically bad. Selling shares at $50 to buy an asset that earns more than $50 of value per share makes the remaining holders richer. Selling shares at $2 to pay salaries makes them poorer. The question is always what the cash bought, which is why the use-of-proceeds section matters more than the headline share count.
Example: a company earns $40M on 100M shares, so eps is $0.40. It issues 25M new shares and earnings stay flat. EPS falls to $40M / 125M = $0.32, a 20% cut, and at an unchanged pe-ratio of 20 the stock drops from $8.00 to $6.40.
Related: secondary-offering, at-the-market-offering, fully-diluted-shares, accretive-dilutive, use-of-proceeds