The word optimize has several meanings related to SEO and online marketing.
One definition is SEO itself; search engine "optimizing" a website, meaning using various techniques to enable it to rank as highly as it can at Google and Bing.
Another definition of "optimize" relates to images; optimizing an image means making it be only as wide (or tall) as it needs to be, for the medium on which it is being displayed. One doesn't need an image that is 2000 pixels wide to display on the screen of a phone. Images also do not need to be of the highest possible "quality" - meaning that one can substantially reduce the file size (number of bytes) of an image by cutting down its quality without degrading what one can see with one's eyes. (Part of technical seo of web pages.)
"Optimize" can also refer to the "optimum" management of online marketing campaigns such as Google Ads; cutting away dross and making sure ads are displayed only to targeted audiences, and that no money is wasted on irrelevant clicks.
Celebrating 25 Years of Creating an Island of Stability In the Sea of Online Confusion