An embroidery hoop is not required for stitching but most people prefer using it. The hoop is used to keep the fabric neat and taut while you’re stitching and it also acts as something sturdy to hold while stitching.
There are many different sizes (and varieties) of embroidery hoops and frames. A smaller hoop can be easier to grip while a bigger one doesn’t need to be moved around as much when you’re stitching. Depending on the size and shape of your finished embroidery an embroidery hoop can be used as a frame (even though it’s actually a tool).
A good tip is that you can get one that works as a frame, if the sizes allow it, and one, a bit smaller that you use for stitching. The hoop you use for stitching can be reused for your next embroidery
To use the embroidery hoop simply untighten the screw at the top so that the two pieces can be separated. When you’re ready to attach it to your fabric, put the smaller ring underneath the fabric and align the bigger ring on top of the fabric, over the smaller ring. Press down the big ring while tightening the screw again. You might need to have a firm grip to keep it sturdy so it doesn’t slip away. You can stretch the fabric a bit as well to keep it extra tight.
When the section inside the hoop is stitched just untighten the embroidery hoop and move it onto a new section.