First we will need a special cable called an OTG (On-The-Go - connecting on the go). He looks like a regular cable: one USB connector-mother, and with other Micro-USB or Mini-Usb. Do not confuse it with the regular USB miniature connector options. Choosing a cable in the store, take a stick and try to insert it into the connector. It can also be identified by the letters OTG on the sticker or the connector.
Insert the OTG cable into Micro USB port of your tablet or smartphone. All Android versions starting with 3.1, supports this cable. Ie if your tablet or smart phone made after 2011, the time this version of the mobile operating system, it needs to support external USB drives.
If all went well, you will see in your smartphone or tablet's storage without any additional steps. It will appear in the list of available drives, for example, in the file Manager, which is included in all versions of Android. Now you can watch movie from external USB drive or copy it to the internal flash drive.