Physics and Geometry
It is when two rays or a line segment can go on and on without touching.
Parallelism refers to sentence structures that include similar word or phrase patterns.
Sentence and list structure
Parallelism is about consistency, and having all the parts in the same form.
In a sentence, compound verbs or other words should be in the same form.
Example : "He sang and was dancing all night" is improper if you actually mean "He was singing and dancing all night". Similarly, "He had finished and went upstairs" mixes two different tenses, and is improved as "He finished and went upstairs" or "He had finished and gone upstairs".
A things-to-do list might read :
Reading to the kids at the library
Singing with the choir
Make sure to buy MILK!!!
Volunteering at the hospital
Parallelism would require changing the third item to "Buying Milk (remember!). You retain the meaning, and have made your list parallel in structure.