I made up my mind--> I have (finally) decided / I am not in doubt anymore / I am sure about this now..

You usually say this to someone who keeps suggesting to you that you should to do certain things or go to certain places. Then you can say, "I can make up my own mind where I want to go and what I want to do."

'To make up your mind' means to come to a definite decision or opinion, often after having considered a number of alternatives.

It mean they already make and decide a conclusion, for example : your friend ask you : Cuccu, what will you go to Bobby house this morning? And you answer : let me think.. (you think which will be best, to go to Bobby house or do some cleaning at your house) Ok I make up my mind (you make a conclusion) He ask : so how is it? You said : I go to Bobby house. <---this is the conclusion.

it's an idiom meaning to decide to do something