WordPress functions wp_enqueue_script / wp_register_script and wp_enqueue_style / wp_register_style add additional version info in the end of the JavaScript and CSS files you link (include or add) using these functions. This version info is important to ensure that the correct version of the file is sent to the browser regardless of caching.
Below screenshot explains how a version number appears in WordPress enqueued scripts and styles, see the highlighted part:
Preventing WordPress to add vers...