Phrase memorization means learning ready-to-use chunks of language — complete phrases and expressions that you can deploy in conversation without having to construct them from scratch.
A related activity is Mad-libs Learning
When you're under the pressure of real-time conversation, your brain doesn't have time to assemble every sentence from individual words and grammar rules. Having a bank of pre-memorized phrases gives you reliable fallbacks: greetings, common questions, polite requests, reactions, and situation-specific expressions.
Phrase memorization is most useful in Phase 4X (Situational Speaking), where you're preparing for specific real-life situations. It's also valuable any time you're about to enter a new domain where you need functional language quickly (travel, a new job, etc.).