Windows 10 integrates Windows Search directly into the taskbar. By default, the search bar is permanently visible next to the Start menu. While this can be helpful, it uses a lot of taskbar space. Nowadays there's also a separate button for the Cortana virtual assistant, which creates further clutter.
You can remove the searchbar and Cortana button for a tidier taskbar. Right-click the taskbar and click the "Search" menu. Choose the "Hidden" option to remove the searchbar. Right-click the taskbar again and click the "Show Cortana button" menu item to hide Cortana.
Alternatively, use the "Search > Show search icon" option to keep search visible but with the text input field collapsed. This mode is handy on touchscreen devices where you can't always use a keyboard to invoke the search interface.
Even with the searchbar disabled, Windows Search will still be available by pressing Win+S or hitting Start and typing. You're not losing any functionality by hiding the taskbar, only freeing up room for more pinned app icons.