If you own shares in a company, you have the right to participate in any new issues of shares unless the shareholders' meeting has decided otherwise. In other words, companies have to offer any new shares to its existing shareholders before they can be offered to someone else. If you for instance own 1% of the shares in a company, you have the right to buy 1% of all new shares the company issues. 

The shareholders' meeting may however decide that the shareholders' pre-emption rights shall not apply for a specific share issue.