Read a book about tractors to the kids. While reading, allow discussions about the books or about tractors among the kids or between one kid and yourself. Discussions may include what type of sounds tractors make or if they have ever seen a tractor. Keep in mind that discussions will vary based on a kid's age and/or learning ability.
Use tractor toys when playing with the kid(s). Make sounds a tractor would make and relate the tractor play to the book you read about tractors.
Post pictures of tractors throughout the room. Pictures of tractors in action are a great way to explain to a child what a tractor does and how they are used.
Continue teaching tractors to kids by doing a tractor arts and crafts activity. The arts and crafts activity can be tied to anything the kid is interested in with tractors. For example, if the kid likes to paint, let him paint a tractor. Or a child can color a tractor coloring book, cut up a tractor picture to make a tractor puzzle, etc.