、。
We dropped our gear inside a little prefab shed and immediately started getting ready.
Literal
We [topic-は] small [attr-な] prefab-hut [gen-の] inside [at-に] baggage [obj-を] put, immediately preparations [obj-を] do.
プレハブ is the loan 'prefab' (from English 'prefabricated'). 小屋 ('small hut, cabin, shed') is a versatile term for small standalone structures. さっそく ('immediately, right away') is a common adverb implying prompt action. The two parallel clauses use the te-form 置き (actually the 連用形 ren'yōkei 'stem form,' functionally equivalent to the te-form in narrative style) to chain actions. 準備をする is the standard collocation for 'to prepare.' Plain-form present tense is typical of vivid narrative prose.