so I rem my doc telling me to watch out for any signs of pre term labor and go to er if any problems.. idk wat to watch…

so I rem my doc telling me to watch out for any signs of pre term labor and go to er if any problems.. idk wat to watch out for. I know pain and blood yes,oviously but do I have to watch out for any other signs..? 😐like wat is the mucus plug I hear everyone talking bout? I know I'm a Lil far but with twins anything is expected so I jus wanna be aware.