I am using definitions from two sites - one delving more into the content while the other describes the 'construction'

Site #1

Tanka is the name of an ancient form of Japanese poetry. Tanka are 31-syllable poems that have been the most popular form of poetry in Japan for at least 1300 years. As a form of poetry, tanka is older than haiku, and tanka poems evoke a moment or mark an occasion concisely and musically. In Japan, the Tanka is usally written as a straight line of characters, but in English and other Western languages, it is usually divided into five lines, with a syllable count of 5-7-5-7-7. Usually, each line consists of one image or idea; unlike English poetry, one does not seek to "wrap" lines in tanka, though in the best tanka the five lines often flow seamlessly into one thought.

With the way the syllable count works, this is like a traditional Haiku with two extra lines added.

Example:

My advice to you
Try the elegant format
That is the Tanka
Stretch your imagination
Let your thoughts take to the air!

Site#2

An unrhymed Japanese poem consisting of five lines of 5/7/5/7/7 (5 onji in the first line, 7 onji in the second line, 5 onji in the third line, 7 onji in the fourth line, and 7 onji in the fifth line) totaling 31 onji.

Onji = syllable