What do you mean I'm in service?

Military service means that the state forces its citizens to perform compulsory labor for a certain period of time according to the law, usually by taking part in labor or joining the army.

After 1, it refers to joining the army after military service. Later, it was extended that some equipment was used in the army.

Because it refers to working for a fixed organization for a certain period of time, it is also widely used to describe an athlete in a team competition.