NFT stands for "non-fungible token", it is a totally unique and non-interchangeable piece of data (image, video, music, object in a video game...), stored in a digital database that uses blockchain technology to establish proof of ownership.
NFTs are collectible digital assets that have value, just as physical art has value, so do NFTs. An NFT is completely unique and cannot be duplicated, and this is where non-fungible makes sense.
Any type of digital file can be stored as an NFT to identify the original copy. You pay for a file and proof that you own the original copy.
In Mechachain, NFTs are the colonies and the different parts that make up the robots called mecha.