Referring to wiktionary, (here: https://en.wiktionary.org/wiki/tack#Noun) The most important definition for you to know is #1 and #2, meaning "a small nail; a thumbtack." This sense can also be used as a verb "to tack (up/on/etc.)" meaning "to add something (onto something else); to attach something (with a tack or in a similar manner)" If you already know that definition, learn #6: "a course of action, or direction" meaning "an approach to doing something." We often use this in phrases, like describing someone as having started something "on the wrong tack," as in, they started doing something "in an erroneous, misguided, etc., way" Don't worry about any of the remaining definitions for now, unless you plan on using English to sail around the world, give horse-riding lessons, or manufacture and market rations for astronauts.