In the United States, what is the difference between lower division and upper division in colleges and universities?
"Lower division" courses are those typically taken by freshmen and sophomores (first and second year students at a typical 4-year bachelor's program).
"Upper division" courses are those typically taken by juniors and seniors (third and fourth year students at a typical 4-year bachelor's program).
Given that you've used the community college tag, it may also be relevant to point out that often community college credits are equivalent to lower-division courses and may be transferrable to a four-year school to substitute, as mentioned in answers to your previous question.
You could also possibly use the terms to group the students into 1st+2nd year and 3rd+4th year students, though these are also often referred to as "upperclassmen" and "lowerclassmen" (I'm not familiar with other gender-neutral substitutes - would appreciate an edit or comment if someone could add one).
There are not typically strong barriers to students taking upper division courses in their first two years (provided they meet necessary prereqs) or lower division courses in their second two years (especially electives outside their major).