Stare decisis — "to stand by decided things" — is the common law doctrine that courts should follow prior decisions (precedent).
The justification: predictability. If the law means what the last court said it means, parties can plan accordingly. If each case starts fresh, the law is unknowable in advance.
The tension: precedent can lock in bad decisions. Plessy v. Ferguson (1896, "separate but equal") was precedent for 58 years. Overruling it required Brown v. Board of Education (1954) to engage in exactly the kind of first-principles reasoning that stare decisis is supposed to prevent.
The doctrine doesn't resolve this tension — it manages it.