Many physical phenomena are repetitive; that is they happen in a regular cycle or with a known frequency. From things as obvious as the rising and falling of the tides to critical phenomena as increase in blood pressure with the systolic-diastolic heart rhythm, these patterns can be modelled mathematically.
To model these, we use the so called “circular” functions – sine, cosine, tangent and their inverse co-functions. These functions are better described a Periodic functions, because while they can be generated from the horizontal and vertical components of a circle, they are not circular, or even wavelike when plotted, but simple up and down movement, or oscillation.
The website, “Betterexplained” has an excellent article about the non-circular nature of periodic functions. Read it!
If you need a review of radian measure, and the use of pi in angles, try this web page.
Here are some supplemental notes that support your development of understanding of applications of periodic functions: