Positioning is basically getting yourself to an airport that is not your home airport or where you really need to go (on a cheap easy flight ideally) in order to catch a flight that is available on points/miles that is available or usually more convenient points/miles wise. Sometimes it is needed because the flight options you have from your home airport aren't good due to schedule, availability or the plane you want to fly isn't located at your home airport.
Some examples of positioning and the varied reasons for them that I have used would be as follows. My home airports are MIA and FLL and I wanted to fly the Emirates A380 plane with the shower in first class to Dubai. That specific plane with that feature does not service MIA/FLL so I had to position to IAH to fly on it. I booked my Emirates flight out of IAH-DXB with Emirates Skyward miles transferred from either Chase or Amex, I don't recall and then purchased a separate cheap ticket on United and flew to Houston. Spent the day with friends over there and then caught my flight to Dubai.
DH, my mom and I were taking a trip to Scotland via London and I wanted seats in F for us. British Airways F seats out of MIA were dry, nothing open even checking as soon as they dropped. I saw 3 F available out of JFK and booked that instead. Plenty of cheap or low miles cost flights to NYC out of Miami so we booked those for the night before our long haul flight and spent the day in New York doing touristy things and used a FN cert for our hotel stay.
More recently, I needed to book 3 flights for DH, niece and I to Greece next year. I wanted to book in business (J) and also have good odds of finding 3 J seats on a specific date that wouldn't be an obscene amount of miles and lower in taxes. My best bet was booking an Emirates Fifth Freedom flight out of EWR-ATH. I will purchase our 3 flights to EWR using UTB funds that have been accumulating for the night before we are flying out. Then I will use FN certs and book our stay and spend the day catching a show and doing touristy things in NY with our niece.
Failure of all intended potential plans to book flights caused me to position to London of all places, later this year as a stopover for 2 nights in order to get to Cairo. We will get to see the NYE fireworks in London and have tickets to see ABBA Voyage. I am also positioning on the way back home. My best option for an easy flight home with miles in J was Egypt Air to JFK. We will stay a couple of nights and catch some shows. Then fly home on United using United Travel Bank funds. So on this trip I am positioning to get there and positioning to get back home as well. Not gonna lie, this epic failure of mine will likely be one of the best travel hacking plans I didn't have. LOL
Hope this helps