A molecule's vibrations are quantised making a ladder of vibrational energy levels. At very, very low resolution a single wide line (or peak) is seen from the $n=0 \to 1 $ transition. At high resolution rotational levels are also resolved as shown in the link in the comment by MaxW above. Additionally, v. weak overtones can be seen from $n\to 2$ and $n\to 3$ transitions (and so at higher energy) due to what is called anharmonicity in the bond's vibration. See a text such as 'Modern Spectroscopy' by Hollas (publ Wiley) for details but many Phys. Chem. textbooks will also explain this.