This is the 11644th most frequent French word.
sache
"Sache" is the subjunctive form of the verb "savoir," meaning "know."
Here, "sache" is the subjunctive present form of the verb "savoir" used in a directive or imperative form to express something the listener should understand or acknowledge.
Quand je parle, sache que je dis toujours la vérité.
When I speak, know that I always tell the truth.
In this case, "saches" is the subjunctive present form of "savoir" used after the expression "Il est important que" to reflect necessity or emotion within a subordinate clause.
Il est important que tu saches que cette tâche est difficile.
It is important that you know (realize) that this task is difficult.
Here, "Sache bien" uses "sache" as part of a directive encouraging the listener to firmly understand or be aware of the statement.