This abbreviation (tt) is used for an ingredient amount and means “to taste”. Tt is commonly used in spices, especially with salt and pepper. Each individual likes different amounts of salt and pepper. Tt allows for you as a home chef to season to your liking.
To taste is also a good way to allow for various types of salt to be used. If you were to use the same amount of himalayan salt as you would kosher salt, the end result would be much saltier with the recipe using himalayan salt.