彼らは海岸をぶらぶらと歩いた。

they (usually male); them

topic marker particle; indicates contrast with another option (stated or unstated); adds emphasis

coast; beach

indicates direct object of action; indicates subject of causative expression; indicates an area traversed; indicates time (period) over which action takes place; indicates point of departure or separation of action; indicates object of desire, like, hate, etc.

dangling heavily; swaying to and fro; swinging; strolling; rambling; roaming; wandering; idly; lazily; leisurely; aimlessly; persisting (of an illness)

if; when; and; with; particle used for quoting (with speech, thoughts, etc.); quoting particle; promoted pawn; indicates question (sentence end)

to walk