we use "be one's side" to mean "stand by sb, support sb".The expression is, "to be on/to take his side/my side" etc. This expression is used when a situation arises where there is a disagreement of opinion

to be on your side = support your side of the argument or battle Next time could you give the example where you see these words?

to support someone, to be on one's side means to be supportive to someone. to be in my team, to be on my side.. to support my actions, words everytime.