An entrant is someone who signs up to compete or participate in something. A person who enters a country is also an entrant, like an American who crosses the border into Canada.
If there are millions of entrants in a lottery, your chances of winning aren't great — but if you're one of only five entrants in your school's raffle, you just might go home with a new bike or a trip to Cleveland. A beginner at some job or activity is also an entrant, like an entrant into the field of microbiology. This meaning is the oldest, dating from the seventeenth century.