so your pocket PCR is a thermocycler, which is a thing that heats and cools samples of liquid to set temperatures for set times for a programmed number of cycle

so your pocket PCR is a thermocycler, which is a thing that heats and cools samples of liquid to set temperatures for set times for a programmed number of cycles. Tube gets hot, tube gets cold, makes a few stops along the way, repeat. Now, what heating and cooling the liquid to precise temperatures a bunch of times actually does depends on what's in the liquid. The most common use case is something called Polymerase Chain Reaction (or PCR, hence the name of the product). As another comment explains, this basically lets you take a little bit of DNA and make a lot of DNA. You can run other reactions as well, such as reverse transcription, where you take some RNA and generate a DNA sequence out of it. There are a lot of reasons you might want to do that, but for example that's a common procedure in diagnostics when testing for viruses like SARS-COV-2. You can do molecular cloning work, which is editing and modifying DNA, stitching DNA sequences together, etc. It's a very useful took and a core component of most biological labs. Unfortunately, if you buy a PocketPCR, you end up with a device that doesn't have a heated lid. This means that you have to add mineral oil to all of your samples before cycling them since otherwise you could screw up the reaction/lose your sample. As someone who has dealt with that before, take my word for it that it's a royal pain. Splurge a bit on the heated lid.