Basically, when a cell divides, it needs to produce an extra copy of DNA. One for each daughter cell. The DNA in your cells is double stranded, which means it is basically two copies of your DNA stuck together. So each one of those copies needs to be duplicated before the cell divides.

You might imagine that the cell would do this by splitting the two DNA strands and sending some molecular machine down each one to replicate it. That is what it does - kind of. See, the tricky part is that the two DNA strands are pointing in opposite directions. The heads and tails of the nucleotides (A, C, T, or G) in each strand are pointed in opposite directions.

You might think that, if evolution can create a machine that works in one direction, it could create a machine that works in the other direction. Then, one could be used on each strand in parallel. Back in the day (~40 years ago), this is basically what everyone assumed must be happening.

But that's not what happens. We only have a machine that goes in one direction. People spent many, many years looking for these little molecular machines, but only ones that went in the same direction were found. None that went backwards.

So for the backwards strand, it's duplicated in a really convoluted process. Basically, instead of copying it all in one shot, it has to repeatedly jump ahead, work back, jump ahead, work back, creating a bunch of little DNA fragments. While it does this, all the little fragments have to be tied together. It's a very strange process.