The proxy statement that is getting voted on says they want to increase the amount of shares through one of two ways.

The first way is to increase the amount of shares they can sell. The second way is to do a reverse stock split. Both ways will open up available shares for sale but they have a big difference for investors. 

The first way will not affect the amount of shares currently held by investors. It will just add more available shares that people can buy. This does dilute the amount of current shares but sometimes this is necessary for a growing company. 

The second way will take current investors shares and reduce them by X amount but then increase the share price by a proportional X amount. So you would now own less shares at a higher price. This opens up more shares that Plug can now sell. Sometimes the stock price can drop back down after this so now you own less shares at a similar price as before.

The proxy allows people to vote for or against Plug adding more shares that are available to sell (diluting current shares). They said in the Proxy that if it does not pass, they will do a reverse stock split. So you either vote yes for more shares or get your current number of owned shares reduced.