As Michael says, 'hold on' is a phrasal verb we meaning more or less "Have patience" or "Keep doing what you are doing for a little bit longer". In fact, in most cases when we say 'Hold on' to someone, we actually mean 'Wait a moment'. 'I'm going out.' 'Hold on a minute, I'll come with you.'

It is a phrasal verb that means something like "Have patience" or "Keep doing what you are doing for a little bit longer". We would say, for instance, if someone was in a hurry to leave but you wanted to go with them and were still putting on your shoes - "Hold on, I will just be a minute".

For the record we also say "Hang on" sometimes to mean the same thing. It sounds a little more casual like "Hang on dude, I'll be there in a minute"