You can use your action to administer first aid to an unconscious creature and attempt to stabilize it, which requires a successful DC 10 Wisdom (Medicine) check.
A stable creature doesn't make death saving throws, even though it has 0 hit points, but it does remain unconscious. The creature stops being stable, and must start making death saving throws again, if it takes any damage. A stable creature that isn't healed regains 1 hit point after 1d4 hours.
{
"srd": true,
"name": "Stabilize a Creature",
"page": 197,
"time": [
{
"unit": "action",
"number": 1
}
],
"__prop": "action",
"source": "PHB",
"entries": [
"You can use your action to administer first aid to an {@condition unconscious} creature and attempt to stabilize it, which requires a successful {@dc 10} Wisdom ({@skill Medicine}) check.",
"A stable creature doesn't make death saving throws, even though it has 0 hit points, but it does remain {@condition unconscious}. The creature stops being stable, and must start making death saving throws again, if it takes any damage. A stable creature that isn't healed regains 1 hit point after {@dice 1d4} hours."
],
"basicRules": true,
"reprintedAs": [
"Help|XPHB"
]
}