As a Logia, this fruit allows the user to turn their body into a sticky substance, making them nigh-invulnerable unless Haki or Seastone is used. The substance is thick enough to be used as a large blunt weapon, and is capable of trapping people, even extremely strong ones, due to its stickiness, especially if empowered by Busoshoku Haki. Its thickness and viscosity is such that large-caliber gunfire is completely unable to pierce through his transformed self.
However, the substance loses much of its stickiness if a sufficient amount of liquid is applied to it, allowing a trapped opponent to free themselves from it; this was first seen when Jinbe used Fishman Karate to douse Katakuri's leg with black tea, freeing the trapped Luffy. Otherwise, the user is affected by the standard Devil Fruit weaknesses.
Katakuri uses his fruit in combat, and is capable of controlling large amounts of the substance with high precision. So far he has been shown turning his leg into it and extending it out extremely fast to form a large, tentacle-like blunt weapon, with much of it imbued with Haki to harm the rubber-bodied Monkey D. Luffy, and at the same time, entrapping him.
The Mochi Mochi no Mi is similar to the non-canon Ame Ame no Mi because they both are Logia that let the user turn into a thick, sticky, syrup-esque substance that lets them constrict their target, and are named after candy.