PHILOLOGOS: The Gematria of Hillary Clinton and Donald Trump. Fun with Hebrew numbers (Mosaic Magazine). Despite the clickbait title and provocative opening, this is a nice introduction to the ancient art of "gematria." This art is based on the fact that each letter in the Hebrew alphabet also has a numeric value. Gematria involves adding up the total numeric value of all the letters of a word or phrase, finding another word or phrase with the same value, and drawing conclusions about the first in light of the second.
Philologos notes that gematria goes back at least to the Talmud. It may be much older. In the late first century CE, the author of the Book of Revelation was adding up the value of a name and using that as a secret code. This isn't precisely gematria, but it is playing with the same ideas.
Many years ago Philologos had another column on gematria in The Forward, but the link to that one has rotted. Other past PaleoJudaica posts on the subject are here and here.