Responsive, Accessible and Mobile-friendly - what do they mean?
This is where a website rearranges itself (through clever coding) to look good on all screens. Older websites will be tiny on a phone for example - you have to zoom in to read anything. A responsive site looks good without zoomig and panning around.
Screen readers are bits of software which read websites out loud for blind or partially blind people. They only work properly when all the images are described (i.e. 'logo' or 'picture of a cat looking amusingly grumpy'), and when important parts of the website like navigation are grouped together and labelled as such.
As well as being responsive, a website is mobile friendly if it doesn’t need to download a huge amount of data to show up. Fancy websites with tonnes of graphics are great on a computer with an excellent internet connection but terrible for those people browsing whilst on the move!
If you are interested in website design there are lots of tutorials and articles out there, but the above attributes of a website are usually figured out through well thought out html and css, so that's a good place to start.
html: hyper-text mark-up language is the basic coding language that websites are written in. Many tools are available to design websites without ever having to learn this code, but a good web designer knows enough to make sites exactly how they want them to look. Most code that makes a website accessible is within the css.
css: cascading style sheets are a type of coding that applies styling to separate html elements. The code can be contained within the html code, or can be saved as separate files and referenced within the html code. Most code that makes a website responsive is within the css.
A website is more mobile friendly by not having slow-loading content, by having 'thumbnails' rather than full size images, or by readjusting itself through css or another coding language when displayed on a smaller screen.