It is a free, free and open-source content management system (CMS). It is written in PHP and is based on an MySQL database. It is the most used CMS in the world. (As of October 2019, it is used by 34.7% of websites in the world) source: Wikipedia.
It is above all a very versatile and easy to learn CMS. With a very large community and a very large bank of free resources. Previously limited to blogging (since its launch in 2003), it currently has many plugins that can answer almost all your problems. Very quick to set up at the start, it significantly reduces the cost of developing a website.
Also many disadvantages
Be careful, I don't spit on Wordpress at all, I myself know people who use it, it is a strategic choice. However, there are many other solutions on the market. In my opinion wordpress is overrated for its ease of use. It can be relevant for certain conditions, but can also limit a project to you in its execution. What can save you time at the beginning can also waste you later. Note also that wordpress is based on exactly the same technologies that are used today on the web. It may be relevant for some agencies to specialize in wordpress. Also to develop a wordpress site for a client who is used to using this CMS, apart from these conditions it can also be the source of many problems which slow down your project if you do not have full control. Some themes can be tricky to install. So it will take you more time to understand the system and set up your site as you wanted. Wordpress is above all coded by different people. In terms of security, it is obvious that the most famous CMS are also those which are the most prey to piracy. Finally concerning performance (and therefore also seo), not being very suitable for adding many plugins, the loading speed of your website will continue to increase (which is harmful for your seo). Not to mention the compatibility issues between extensions that can waste a lot of time.
Wordpress for learning to code
If you are learning to code, in my opinion it is a mistake to use Wordpress for the long haul. You might as well learn to use native functions.
I do not use wordpress personally because I took the part of developing my own CMS for my business. It has the advantage of fully adapting to my needs and priority being given to performance, especially in terms of display speed and visibility. The technologies on which my CMS is based remain exactly the same.