Stamina and physical endurance.
Constitution affects your maximum Hit Points and Concentration Saving Throws.
Your Constitution modifier is added to the number of Hit Points gained per level (including negative values), and is applied retroactively.
Constitution Modifier Chart
Baldur's Gate 3 uses the Point Buy ruleset.
| Ability score | Modifier value | Ability score | Modifier value | |
|---|---|---|---|---|
| 1 | -5 | 16-17 | +3 | |
| 2-3 | -4 | 18-19 | +4 | |
| 4-5 | -3 | 20-21 | +5 | |
| 6-7 | -2 | 22-23 | +6 | |
| 8-9 | -1 | 24-25 | +7 | |
| 10-11 | +0 | 26-27 | +8 | |
| 12-13 | +1 | 28-29 | +9 | |
| 14-15 | +2 | 30 | +10 |
Related Items
Increases Constitution "to"
Amulet of Greater Health sets 
to 23
Increases Constitution "by"
Dwarven Splintmail increases by +2 (up to 20)
Gloves of Soul Catching increases by +2 (up to 20)
Enraging Heart Garb increases by +2 (up to 20)
+1 to Saving Throws
Helmet
Flaming Fist Helmet
Dark Justiciar Helm
Helmet of Smiting
Holy Lance Helm
Flawed Helldusk Helmet
Dark Justiciar Helmet
Sarevok's Horned Helmet
Advantage on Saving Throws
Spidersilk Armour
Barkskin Armour
Drunken Cloth while 
Dark Justiciar Half-Plate (Rare)
Steelwatcher Helmet- Amulet of Greater Health
Dark Justiciar Half-Plate (Very Rare)
Moon Devotion Robe
Armour of Landfall
Other
Gibus of the Worshipful Servant 
Advantage on 
Saving throws
Elixir of Half-Elven Healing Regain hit points equal to your Constitution score