Hun grootste prioriteit is nu om veilig thuis te komen.

Yes. In Dutch, hun is the possessive pronoun corresponding to zij (they), so hun grootste prioriteit literally means their biggest priority.
Note that hun can also appear colloquially as an object pronoun (Ik zie hun), but the standard object form is hen. In this sentence, however, hun is correctly used to show possession.

The om … te construction introduces a purpose or intention, much like English to in to get home safely. Its structure is:
• om (introduces the infinitive clause)
• any adverbs or objects (here veilig thuis)
• te
• the main verb at the end (here komen)
So om veilig thuis te komen = to get home safely.

Nu is an adverb of time. In a Dutch main clause, adverbs typically come right after the finite verb (is). Hence is nu om …. You could also say:
• Hun grootste prioriteit is om nu veilig thuis te komen.
Both are correct, but the emphasis shifts:
– is nu om → now the priority exists
– om nu veilig → now the coming-home is the priority

As a bare infinitive, you’d normally write thuiskomen (one word). But when you insert te before the main verb, you separate the adverb thuis and put te right before komen:
• om veilig thuis te komen
Think of thuis as an adverb of place, not a prefix, so it stays separate when te intervenes.

Dutch forms superlatives by adding -ste to the adjective root. For groot (big):
• Positive: groot
• Comparative: groter
• Superlative: grootste
Since it stands before a noun (prioriteit), you keep the extra e: grootste prioriteit.

Prioriteit is a feminine noun borrowed from Latin/French. Plural: prioriteiten.
Pronunciation tips:
• Stress on the second syllable: prio-ri-teit
• The -iteit ending sounds like English -ity (e.g. prior-ITY).