A call to action (CTA) is a phrase that calls out your viewers to do something. 'Please subscribe', 'Click the like button' 'Watch another video over here' are all CTA's. Within the YT community there are people who really hate YT related CTA's (sub and like requests) at the beginning of videos and will instantly dislike (as you can see in this experiment). However, the human psyche does work in a way that CTA's 'remind' people to do these things, so doing them at least somewhere in your videos is preferable.