Natsar (נָצַר) means to watch or guard with attentive care — the kind of vigilant preservation that keeps something from harm or decay. It describes a sentry watching a city, a shepherd guarding a flock, and a disciple keeping commandments. The root is related to the word for 'watchman' and appears in the name 'Nazareth' (Netseret) — a connection Matthew exploits in 2:23.
Theologically, natsar appears in the crucial passage Exodus 34:6-7, where God declares His character: 'The LORD, the LORD, a God merciful and gracious, slow to anger, and abounding in steadfast love and faithfulness, keeping (natsar) steadfast love for thousands, forgiving iniquity...' God's own self-description includes being a keeper/guarder of covenant love. Psalm 31:23 calls believers to love the LORD, who 'preserves' (natsar) the faithful. Isaiah 42:6 applies it to the Servant: 'I will... give you as a covenant to the people, a light for the nations, to open blind eyes, to bring out prisoners from the dungeon, from the prison those who sit in darkness.'
'Keep your heart with all vigilance (natsar), for from it flow the springs of life' (Proverbs 4:23) — perhaps the most intensive use of this word's spirit. The one who watches attentively over the heart guards the source of all life. God's own keeping of His covenant love (Exodus 34:7) is the foundation and model. Because God is the ultimate Watcher who never sleeps or slumbers (Psalm 121:3-4), those who are 'kept' by Him can trust their security and imitate His watchfulness.