What's the difference between 'Join' and 'Commit to Join'?

Selecting the "Join" option means that you will purchase the item regardless of what price point is reached. 

The "Commit to Join" option is only available on drops where the lowest price point has not yet been reached. By selecting this option, your order will only be placed if enough people join the drop to reach the lowest price point. 

You can change your joining option at any time as long as the drop is still active.