There's no difference in meaning, it's just an arbitrary grammar rule. "Most" is used before indefinite plural or indefinite uncountable nouns. "Most of" is used before "the" (i.e. before definite plural or definite uncountable) nouns, pronouns or possessive pronouns.

E.g.

"Most people think that..."

"Most of the people here think that..."

"Most of us..."

"Most of them..."

"Most beer is..."

"Most of the beer is..."

"Most of their friends..."

You cannot say most them

most of: come in front of a noun WHITOUT  a determiner (modifying word)
eg: Most students
most : comes in front of a determiner (the, my, this...) or a pronoun (us, them)
eg: : most of THE students,
most of us

you cannot say most them

most of: come in fronmt of a noun WHITIUT  sterminer (midifying word)

eg:  Most students  

most : comes in front of a determiner (the, my, this...) or a pronoun (us, them)

eg:  : most of THE students,  

most of us,

we can say most of them. And can I say most them？