The bobbin in the feed dog part of the sewing machine makes up the bottom of the thread needed for sewing.
Huge numbers of sewing machines have been manufactured over the years.
Run your finger over this and if it feels rough (probably due to needle strikes) then smooth off any burrs with fine emery. In the case of a second hand machine, make sure that all the bobbins that came with it (and any extra ones that you buy) are of the right sort.
There are several types, superficially similar, and the wrong ones will cause problems if they work at all. The bottom mechanism is accessed by tipping it on its side as in the previous step, and the top mechanism can generally be accessed by removing a couple of screws retaining the top cover.
Nevertheless, the life and effectiveness of any machine can be extended with simple maintenance, and my aim here is to show you how.
The first thing to do is to remove all dust and fluff, wherever you can find it.
On a modern plastic machine you will generally have to remove a number of screws, and you may then have to release a series of clips holding 2 halves of the plastic case together.
Probe the crack between them with a thin spatula or blunt scalpel or thin knife in order to determine where the clips are.
Between the feed dogs is a favourite place for it to collect.
Remove the needle, the foot and the needle plate to facilitate cleaning.