The words "alumnus" and "alumni" both refer to graduates of a school or university, but they have different meanings and uses.
"Alumnus" refers to a singular male graduate of a school or university. For example, "John is an alumnus of Harvard University."
"Alumni" is the plural form of "alumnus" and can refer to a group of male graduates or a mixed group of male and female graduates of a school or university. For example, "The alumni of Yale University held a reunion over the weekend."
Note that when referring to a group of female graduates, the term "alumnae" is used. For example, "The alumnae of Smith College held a fundraising event for the school."
In summary, "alumnus" is used to refer to a single male graduate, "alumni" is used to refer to a group of male graduates or a mixed group of male and female graduates, and "alumnae" is used to refer to a group of female graduates.