"13=UFS" is an anagram-style brain-teaser that translates to the number 13 being unlucky for some. Brainteasers such as this are often used in intelligence tests such as Mensa as well as cognitive therapy.
In this style of brain teaser, numbers and single letters are arranged in a way that prompt the participant to form a complete sentence or phrase. Mensa uses similar brain teasers on its intelligence test with some of them being very complex. An example of one of the longer teasers is "32 is the T in D F at which W F," which translates to 32 is the temperature in degrees Fahrenheit at which water freezes.