When you come across an unfamiliar word, you have to look it up in a dictionary.
Literal
Unknown's word [with-に] meet time [at-には], dictionary [with-で] look-up must.
Several things worth pointing out. 出会う ('to encounter, come across') takes its object with に — the thing met is the target of encountering, not a direct object. 時には ('when,' 'at the time of') is a set temporal frame, slightly more formal than plain 時. And ~なければならない is the obligation pattern — 'must X,' 'have to X' — built from the negative conditional ~なければ + ならない ('won't become'), literally 'if not X, it won't do.'