Floats were never intended for layout, however clever developers found ways in which to use them to create their layouts. Having worked with floats you may have noticed some limitations and bugs (clearfix anyone? vertical centering?).
Flexbox is a new layout mode in CSS3 initially developed by Mozilla to make layouts. There were 3 different iterations of the flex property, the final 2012 spec —
display: flex has excellent browser support. The current Flexbox spec is fully supported in all modern browsers, including mobile, IE11 and Edge.
While watching video #13 on the Flexbox series and coding along, you might find that your are not getting the same results with what is being shown on the video. This happens because a lot of years have passed since this video was uploaded (2015) and browsers have evolved to fully supporting Flexbox. Therefore, Autoprefixer will not produce the vendor-prefixes. Don’t worry for that but make sure to understand how Gulp works, as it can work equally well on other not-widely supported CSS properties and it is one of the most powerful and popular build tools available for front end developers.
Once you have completed the course you can read through CSS-Tricks Complete Guide to Flexbox should you want more in depth information.
Read through Interneting is hard’s tutorial to see how to use Flexbox to help create flexible layouts.