Tax added to imported goods to make them less competitive that locally manufactured ones. If country A makes widgets at 10 groats per widget and country B can make them at 5 groats each then someone in A can import them paying 6 groats per widget, sell them at 7, still make a profit and undercut local widget makers.
Government of A slaps a 4 groat tariff on imported widgets and they're now 11 groats making them less competitive than the local ones. People start buying local widgets. That's the theory but it assumes that A still has a widget making industry or one that can keep up with demand if not then the good folk of A are now paying 11 groats per widget.
And of course in the real world the USA slapping tariffs on China means that China will respond and of course the USA imports a lot of stuff that it doesn't/can't make itself such as TV's or rare earths from China, or oil from Canada or components from Mexico which go into American goods making them more expensive. The importer pays the tariff since despite Shitgibbon's gibbering there is no mechanism for making the originating country pay.
The importer then has to swallow the tariffs and possibly ultimately going bust or passing them onto the consumer making stuff more expensive.