I have to make a magazine for school and it said a tagline.What is a tagline??????

Here is a terrific explanation, along with wonderful examples of taglines:

http://en.wikipedia.org/wiki/Tagline

ok but i cant understand that tell me in easier words

If the tagline is for your magazine, you'll need to think of a short sentence or phrase that describes your magazine. For instance, the tagline of the New York Times newspaper is: "All the news that's fit to print."

"The idea behind the concept is to create a memorable phrase that will sum up the tone and premise"

A "memorable phrase" is often an advertising trick that is included in all of a company's ads so that you and all others out there will associate the slogan with their product.

Dow Chemical (maker of an incredible array of plastic and other products in our lives) used to have as a tagline, "Better living through chemistry."

Did you read those movie taglines in the link I sent you? Now think of a bunch of companies (McDonald's, Burger King, Walmart, etc.) and ask yourself what wording (phrase or sentence) is always connected with their ads.

A tagline is a short and memorable phrase or sentence that sums up the tone and premise of a magazine, company, or product. It is often used in advertising to create a connection between the slogan and the brand or product. For example, the tagline of the New York Times newspaper is "All the news that's fit to print." It is designed to leave a lasting impression and make people associate that phrase with the newspaper. Similarly, companies like McDonald's, Burger King, and Walmart have their own taglines that are consistently used in their advertisements.