Here, let me Google this for you.

MIL-STD-810 - Wikipedia

If you have further questions about this and need clarifications on points, ask me in the comments.

Edit:

Explaining the Wikipedia article
(also found in the comments to Sadaf, below.)

Ah! I see what you mean. It is a little dense, isn’t it?

Okay, so what MIL-STD-810G is - it’s like the Halal signs on some foods. It signifies that a particular product has met a given set of criteria. It’s easier to use “MIL-STD-810G” than to publish the results one-by-one (takes up less space on specification lists), and also because individual products will have some variance in the results of each criteria (but not enough to fail meeting the criteria).

In this case, the criteria mean:

• This product has been through destruction testing.
• i.e. where they put a smartphone through all kinds of fun things, like fire, water, overcharged electricity, salt fog (to test if the smartphone rusts), what happens if you use it on a beach and sand gets onto or into the casing, etc.
• Destruction testing to a military grade. Which I’d like to think (personal opinion) means they’ve given it to a soldier and said soldier has done their best to break the thing (under semi-recorded conditions), at least compared to one of the old Nokia bricks.
• The reason it’s done in live testing is because lab testing alone doesn’t have the same effect as using it live - some people will get into unique situations and also treat the phone like crap, thus resulting in different things happening than in a controlled, professional, careful lab.