Its an online training platform where you can enroll for the courses for free. They also provide the certifications once you have completed the course and have given the exam but if you want the certificate from them then you would need to pay.
The fact that these high quality training is for free, so you should check the courses. The website is clean and very easy to use and understand and hence no complaints on that.
It a very good platform to learn more and expand your current knowledge. With so many great mentors out there it worth spending time and checkout what it has to offer.