A font is a set of letters, numbers or symbols that share a unified design. The design is called a typeface.
Put simply, the font is the style you apply to the copy in your design. There are masses of fonts on the market to choose from which can help stylise your work.
A word of caution on fonts: Fonts are like imagery in that they are subject to copyright. If you have a license for certain design platforms such as the Adobe Suite you will have access to the fonts for as long as you hold the license. In other words, you pay to play.
There are also a huge number of free Google fonts you can use so don’t get caught out using something you shouldn’t, if in doubt, go for a free font.