You infringe a trademark when you use it in a way that allows confusion between your goods and services and their goods and services.
Trademarks are limited by geography (although global brands are ... global) and industry. You can use the word “apple” to sell, say, apples or plumbing services, but not computers or consumer electronics. You can also use the trademark Apple to identify goods and services made by Apple - because that’s what a trademark is for. You can’t use it in a way that people could be confused that your goods and services are their goods and services or are related to or endorsed by them.