Trademark is a very wide term. As the USPTO defines it, A trademark can be any word, phrase, symbol, design, or a combination of these things that identifies your goods or services. In this context you might get the impression that you are able to register practically everything as a trademark and obtain protection. Actually, there are restrictions that apply. The main one is that trademarks can't be functional, meaning they should't affect the performance of goods or services that they are protecting. If that's the case, the trademark is non-registrable and you are more likely to obtain a patent instead of a trademark.

Check out the following limitations on what the federal government will uphold as a trademark and allow you to register it as one:

➡️ Generic trademarks – Words or phrases that are commonly used, cannot be trademarked. Take the example of a footwear store; you cannot trademark the word “shoes” in order protect your brand. What’s particularly interesting here is that if you run a fruit store, you wouldn’t be permitted to trademark the word “apple.” Meanwhile since that name has nothing to do with the tech industry, Apple can hold a trademark to sell phones and computers under that name.
➡️ Existing trademarks – This is quite an obvious type of limitation. Word or phrases that are already a registered trademark cannot be trademarked by someone else, but only if you are operating in the same field of products or services. Two businesses selling cars can’t both hold the trademark with the name “Jaguar Car”, but the USPTO could allow you to use the word jaguar in another field of business, for example in footwear business to protect a brand of shoes. Here however you have to prove that this trademark is not likely to cause customer confusion.
➡️ Some unregistered trademarks – The state laws recognize and protect trademarks even if they have not been registered. Some businesses are satisfied with a registration in their state which gives them some local trademark law rights. If you try to register with the same name, there will be a determination based on whether having two businesses with the same name will cause consumer confusion.