I am using Canon 70d and there is a small camera symbol in my viewfinder. I dont know what that is or what it's showing. When I click the AF button, it's started blinking.
This is the electronic level indicator. It indicates that you're not holding the camera parallel to the ground but instead have tilted it. The specific configuration indicates that you've got a tilt of greater than 2°.
From page 66 of the manual:
Note that this is configurable — you can turn it off if you don't like it. It's also available on the rear LCD during Live View shooting (where it's maybe a little more obvious as a level).
Also note the manual's warning of a 1° margin of error (or even greater when the camera is far from level).
Its a symbol representing the output of the electronic level. In this instance, the camera is tilted to the left.
See page 66 of the manual