Have you enabled USB Debugging?
Enable USB debugging on your device.
-- On most devices running Android 3.2 or older, you can find the option under
Settings > Applications > Development.
On Android 4.0 and newer, it's in Settings > Developer options.
-- Note: On Android 4.2 and newer, Developer options is hidden by default.
To make it available, go to Settings > About phone and tap Build number seven times.
Return to the previous screen to find Developer options.
For more information go through this developer's website.
If you are on linux then
Very clearly answered For Linux you need to setup UDEV to support the manufacture's identity, or you need to restart adb using sudo.
To do it correctly, do this:
lsusb
and look for your tablet:
Bus 002 Device 008: ID 04e8:6860 Samsung Electronics Co., Ltd Note the ID.
Then go to
/etc/udev/rules.d
and create a new file called androiddevelopment.rules in that file put this:
SUBSYSTEM=="usb", ATTRS{idVendor}=="xxxx", OWNER="yyyy", GROUP="zzzz"
Where xxxx is the first part of the ID (04e8 in my example), yyyy owner is your username and zzzz is your group.
Then restart udev:
sudo service udev restart
That should allow Linux to automatically allow your user to connect to that device.
Got this answer from Reference.