Other forms: personalized; personalizing; personalizes
To personalize is to design something (or change it) so it's distinctly suited for one specific individual. You can personalize your locker by taping up family photos, or personalize your water bottle by writing your name on it in permanent marker.
The verb personalize is often used to mean "make to meet your specific requirements," so a hotel might personalize your visit by leaving a note with chocolates on your pillow, or a barista might personalize your latte with a design on top that's specially made for you. You can also personalize things by putting a person's name or initials on them: "I bought letter patches so we can personalize our backpacks."