When a company is incorporated the founders set up the ownership structure. This structure may include a vesting schedule for founder's shares. The reason for the vesting schedule is to assure future investors in the company that the founders are in it for the long haul. A typical vesting schedule that isn't too aggressive and the one I used in the case of my current company is to vest 25% up front and the remainder in 1/60 increments per month over the next four years.
When taking in institutional money, the new nvestors will assess the percentage of vested shares the founders have and may ask for a reset on some percentage of these shares. One way this happens is that they will add a "repurchase right" term to the term sheet which enables the company to repurchase a certain amount of the founders shares at a nominal price for a period. This number of shares will diminish over time, essentially creating a new vesting schedule for founders.
My advice is that if you know you will be taking in money, find a happy middle ground between something aggressive that will undoubtedly be reversed and a vanilla four year vesting schedule. Starting the company is certainly worth a chunk of up front vesting.