What do you mean by Anti meridian and post meridian?

Ante is the Latin for "before"

Post is the Latin for "after"

A meridian is an imaginary line running through some point on the earth's surface connecting it to the north and south poles. The definition of mid-day is when the sun is directly over this line. This word takes it name from....

Dies is the Latin word for "day' (diem in the possessive case)

meri is a Latin prefix for middle

meridiem is therefore Latin for "middle of-the-day"

Ante-meridiem is a Latin expression meaning "before mid-day"

Post-meridiem is a Latin expression meaning "after mid-day".

Note: there is a difference between Anti-Meridian and Ante-Meridiem. The Anti-meridian is the 180th meridian. (Both 180 degrees west and east.) Which is the basis for the international date line.