Many words are commonly used in ordinary language. Can they become trademarks? They can if they acquire special significance in reference to particular goods. For example, “apple” is a common word, but also a trademark for computers and for recordings. The word has acquired “secondary meaning” in each product category because consumers associate it with a particular brand of product.
Common words can be protectable trademarks if they are used in arbitrary or unusual ways. One cannot trademark DIESEL to sell that generic type of fuel, otherwise no other diesel fuel dealer could use the word to identify the product. However, one could trademark DIESEL as a brand of ice cream. The owner of the ice cream mark can’t use its rights to prevent fuel dealers from using the word on their station pumps nor can it prevent anyone else from using the word for non-trademark purposes, such as a website listing diesel fuel dealers.
Secondary meaning gives trademark owners protection, but does not prevent people from using the same word for other types of products or in common conversation.