A copy of the Samsung Odin software which is a tool for Windows used to flash firmware to Samsung devices running Android. It can be used to downgrade/upgrade or factory reset your phone to stock software, provided you have the correct firmware. It might also fix a boot loop or bricked device.

The type of firmware file required is different depending on how recent the phone is; firmware can be found at https://www.sammobile.com/firmwares/ (although there is a speed limitation) and also at https://updato.com/firmware-archive-select-model/ which can be sped up with a download manager that uses multiple connections. It is also possible to use these websites to find the firmware name you need and then search for it on other websites such as XDA Developers.

The original software can be found at https://samsungodin.com/ and instructions are also on that website, although you should search for instructions specifically for your model.

Some further information about firmware names from https://www.droidviews.com/how-to-change-csc-in-samsung-galaxy-phones/:

All Samsung Galaxy Series firmwares have three components that are as follows:

1. PDA
2. Phone
3. CSC

The name of a firmware contains all these details. Thus, PDA refers to the build version of the software/firmware, Phone refers to the baseband/modem version, and CSC (Consumer Software Customization) denotes to a specific geographical region or carrier. The CSC code contains the info about software packages specific to a particular region, carrier branding and also APN (Access Point Name) settings for data connection.

In this way, unless the PDA, PHONE and CSC codes do not match correctly, you will not be able to update your device over Samsung Kies or via OTA.

Now remember that the product code and firmware version differs from one device to another according to the region or country where it is sold. In most cases you can find out the country or region of a device for which it has been made by referring to the firmware version.