Briefly put, manually keystroked means that you must type the entry by hand. In other words, you can't use an automatic entry service, a program like Roboform, or even the auto-fill feature of your internet browser to enter.
Whether sweepstakes sponsors can actually tell if you type your entry by hand is a matter of much debate. If you don't want to take any chances with disqualification, however, take the time to type sweepstakes that require manually keystroked entries by hand.
To check whether sweepstakes have manual keystroke requirements, you can do a quick search of the rules in most cases by opening them up, pressing "CTRL-F" to open your browser's search function, and typing in a keyword. I usually scan for "man" to cover variations like "manual entry" or "manually entered."