Description Metatag in Google

Use of Description Metatag in Search Engines

What happens if i don't put a Description metatag in my webpage??

Search Engine Google usually displays the sentence in your description metatag when it displays search results on it result pages  but if there is no description metatag on your page than it will use the first few sentences of your page as description or will use the sentence that surrounds the searched keyword on your page.

However its always advised that you put description metatag on all pages and all should have different descriptions within them.