You've told us virtually nothing about your hardware, your OS, your AE version, your project or your output settings, so nobody can do much more than stab in the dark.
The most common cause of stuttery playback from AE renders is the use of unsuitable video codecs. Typically, inexperienced users render files that are simply too big for their hard drives to play back.
Look at this question in the forum's FAQ section:
|FAQ: Why is my output file huge, and why doesn't it play back smoothly in a media player?|