Semrush is a search engine data tool. With Semrush you get insight into how well a website ranks in the search results of Google. Semrush offers insight in search engine optimisation (SEO), link building and search engine advertising (Google Ads). Furthermore, Semrush offers various tools to optimise these scores.
Semrush is widely used for online marketing purposes, especially for SEO work. With Semrush you have insight into, for instance, the search position of keywords. You do not only have insight in the scores of your own website, but also in the scores of the website of a competitor. For this you only need the URL.
Semrush compares the scores of your website with the scores of the websites of your competitors. Based on this data you can improve your website to end up higher in the search results than your competitors.
Other search engine data tools are for example Moz and Ahrefs.