Gels are thin coloured plastic sheets that are transparent. You can put them in front of your flash. You can find them for use with your speed lights, but also you can find bigger sheets, to be put on your studio strobe. Light modifiers like barn doors or snoots often come in three or four different colours. You can find gels in sheets in a size like A4.
I like using them, for the effect of a light, in some colour, cast onto the background. I have found out that a spot on the background in a colour looks really great. I learned that the red coloured one that came with my snoot didn't look good on a black background.
The others do however. And it looks great in a very small angle to a black background, casting a strip of light down across the background.
But try placing a spot in the top corner of the picture, or behind the subject's head. Or try moving the light around to see what effect you can make.
The set of gels that I bought was shipped with a system that is attached to the speed light with Velcro, having a plate where the gel slides into place and that plate can then be flipped down so it is in front of the speed light, or flipped up if not to be used.
That system is really clever. The brand name is Falcon Eyes, bought via Amazon.
Here you can read about some gels I got in the size of about A4.