What is a 13d?
The 13d filing is a report that institutions file to the SEC when they purchase more than 5% of the outstanding shares of a stock. The report lists some details about the transaction included its date, number of shares bought, and the reason for buying the shares.
Sometimes called the schedule 13d, the report is often filed by institutions seeking to engage in shareholder activism, thus investors tend to watch 13d filings more closely than 13g filings.
A 13d filing is a form that funds submit to the SEC whenever they acquire more than 5% of a company with the intention of influencing the company's decisions.
That intention is often called activist investing, as opposed to passive investing where the fund simply purchases stock and leaves the decision making to the company's management.