Constitution is a measure of your character’s health, stamina, and general hardiness. This directly affects the amount of hit points that you gain each level. Plus, attempting to resist the effects of poisons and other such devious things will often require a Constitution Saving Throw to resist.
Having a high Constitution Ability Score means that your character is in great health and possesses a great deal of stamina. These characters almost never get sick and are the pinnacle of great health!
A character with a low Constitution score, however, is particularly susceptible to poisons, diseases, and enemy attacks. With a lower pool of hit points, these characters at the greatest risk of dropping in combat.
While it is generally not advised to put your lowest score in Constitution, it can make for an interesting character. Perhaps your Wizard was chronically ill growing up. While they still battle this illness, they spent countless hours reading about the world outside their home before becoming an adventurer.