In a secondary transaction, existing shareholders (founders, employees, early investors) sell their shares to other investors. The company itself raises no new capital; it is about providing liquidity to current holders.
Where in the journey
Any, but common at late-stage and around IPOs.
What the money is for
Give early shareholders liquidity without a new primary raise.
Common investors
Secondary funds, growth/crossover investors.
Watch out for
Pricing can be opaque; large secondaries may signal insider sentiment.