The 83b form is an option the IRS implemented which allows you to pay taxes based on the initial value of the Units issued by a Company, which is generally $0!
If you don't file this form then, you have to pay taxes on the shares as they vest overtime. This means, if your company is successful, that you might suddenly be hit with a giant tax bill a year or two down the road.
The 83b form needs to be filed within 30 days after signing the post-incorporations documents. If you have vesting on your Shares, PLEASE PLEASE REMEMBER TO DO THIS!!!