What does it mean if you put a video into after effects and the video plays in slow mo
It depends upon your PC environment and the codec. I would suggest that you try to play the video by another codec or update your graphic card driver.
Sorry but I'm new to pc and I don't know how to do those. can you tell me how to?
Playing in real time in after not possible all the time and most time AE is required to cache the frames (green bar) before playing any preview. so I highly recommend following some essential course for AE to get understand how this software work. try https://www.linkedin.com/learning/ where you can find a lot of helpfull cources