Push fittings for 3D printers

Push fittings are one of the classic wear parts of a 3D printer. They serve as a connecting element to the Teflon hose and are located on both the hot end and the extruder of the 3D printer. Inside the fitting are small teeth that grip the hose and hold it securely. The name "push fitting" is based on the feature of being able to open these teeth when needed by pushing on the fitting and quickly inserting and removing the tubing.   

What should I look for when choosing a push-in fitting?

There are basically only two things to consider when choosing the right push fitting: the thread size and the diameter of the fitting. You can find out the thread size by looking in the manual of your 3D printer. In most cases, this is an M6 or M4 thread.
The diameter of the required fitting or hose inlet is determined by the filament you are using. Typically this is 1.75mm.