That phrase comes from the idea of a puppet on strings. The person who controls the puppet does so by pulling on the strings to make things happen. (At least that's where I think the phrase originates.) Similarly, if someone 'pulls a few strings', they most likely used their power or position to make something happen - generally something that didn't seem possible at first. Let's say that I have a friend in the government, and I am trying to get a VISA. However, I am unable to get one. I could contact my friend in the government and ask "Hey, is there anything you can do to help me? Maybe pull a few strings and get me my VISA?" Here, I am assuming that my friend can use his power or position to help me out.

If you have bothered to look it up in a dictionary, you would have found this authoritative definition, "make use of one’s influence and contacts to gain an advantage unofficially or unfairly". (Oxford) If you rely on a good dictionary, you will benefit from accurate and good-quality definitions. By today's standards, this expression carries an implication of unethical behaviour. So be careful how you use it. It could cause immense offence. It could also give people an unfavourable impression of you.

Pull a few strings is really hard to explain hahahha. It's like doing something to get something done that might not have happened or would have taken very long to do. That wasn't a very good explanation ha. Here's an example: " I forgot to hand in the form for the school trip, so my mum had to pull a few strings for me to go." Probably not much help, sorry,