'Tis is an old-fashioned contraction of “it is,” commonly used in poetry and older English writing. It's similar to 'twas, which means “it was.” You might recognize that one from the famous Christmas poem that begins, “'Twas the night before Christmas.”

Writers used contractions like 'tis and 'twas to make their verses flow smoothly and fit the rhythm of their poems. Shakespeare used these words constantly in his plays.

Today, people rarely use 'tis in normal conversation, though you'll still find it in older books, poems, Christmas carols, and sometimes when someone wants to sound playful or old-fashioned. If your friend asks whether it's raining outside, you probably wouldn't answer “'Tis raining!” unless you were joking around. But when you read classic literature or poetry written before the 1900s, you'll see 'tis everywhere. Understanding that it simply means “it is” helps these older texts make much more sense.