To enter is to go inside or into a place. When you enter your house after school, you might pet your cat and take off your shoes.
You might enter the kitchen to see what's for dinner, or enter the library to look for a book to read. In both cases, you go from one place into another, through a doorway. If you're an actor, the word enter takes on a different meaning: to appear on the stage during a play. The stage directions might read, "Enter the King, stage left." Countries also enter wars, and people enter contests — they register or become involved.