Investor is a Perk that becomes available when you reach 70 in the Speech skill. Using this Perk allows you to invest 500 in certain shops and merchants.
This means that the shops you have invested in will have 500 more with which to barter.
You may also invest in the shop that your spouse owns.
As a general rule, you could make an investment only once to each shop, though there are some exceptions (read below).
PS3 For those who prefer to have as little 'Days Passed/Hours Waited/Slept', restarting the game (must exit to the PS Start Screen, not the game's Title Screen), then selecting Continue; one can restock any Merchant's funds,rendering this perk as well as Master Trader redundant (Merchant's funds revert to previous values pre-restart if any saves are loaded, excluding the first Continue.
PC PS3 360 Save game, then kill a merchant. Reload the game and the merchant will be fully stocked with new items and funds (this works without the perk).
PS3 After installing patch 1.9 get the investor perk, invest in every available merchant, increase your speech skill to 100 and convert the speech skill to Legendary. Despite returning your perk points, the investments remain and merchants will have more money to deal with you. This also works with the Master Trader perk which together with investor will give most merchants approximately 2,500 coin making it much easier to level speech back to 100.