FTIR is a method of characterization of a particular material. It stands for Fourier Transform Infrared spectroscopy.
In this spectroscopy, IR radiation is passed through the material. Now in every molecule, there are different kinds of vibrations already taking place like stretching, bending etc. Each kind of vibration has a particular frequency. When we externally pass IR rays, then some of the frequencies of the external IR radiation matches with the frequencies of the vibrations happening inside the the material. Those particular frequencies are absorbed by the molecules. The frequencies that do not match with any of the vibrational frequencies are not used and they are transmitted through the material as it is.
Now we place a detector at the end of other side of the sample. The frequencies that are absorbed by the material are not detected by the detector and the ones that are transmitted through are only detected by the detector.
When we plot a graph of the transmitted output spectrum (intensity vs wavelength or frequency), the frequencies that have been absorbed by the material can be seen as a dip in the graph. When the frequency values at the dip points are compared with standard results, we come to know, which particular frequencies have been absorbed and what exact kind of vibration they cause within the material.