Lines 1-4 establish the poem's setting and dramatic situation. "You," the main character, are one of the players in a game of hide-and-seek. The speaker, addressing you in the second person, is advising you on how to hide without getting caught. (Presumably, this means you're supposed to be a child—although the second-person narration invites readers to imagine themselves in the situation, no matter how young or old they may be.)
As the poem begins, you've already chosen your hiding place: behind or underneath some heavy "sacks" in a "toolshed." Really, then, the speaker's advising you on how to stay hidden: how to be quiet and unobtrusive enough to avoid detection.
From the start, this game is a bit unusual. Although many variations of hide-and-seek are played around the world, the most common versions involve one "seeker" looking for multiple "hiders" or multiple "seekers" looking for multiple "hiders." Often, the seeker will announce the start of their search by yelling a phrase like, "Ready or not, here I come!" Here, however, a group of seekers is looking for a single hider—you!
Moreover, they don't announce that they're ready to start looking; instead, the speaker tells you to announce that you're ready to be found:
Call out. Call loud: 'I'm ready! Come and find me!'
In real life, this yelling would risk giving away your hiding place, or at least giving a clue as to your whereabouts. Yet in the poem, you seem strangely confident that you won't be found: "They'll never find you in this salty dark." (Still, the speaker warns you to "be careful that your feet aren't sticking out," in case the others do stumble on your hideout.)
One possible reason for these strange circumstances is that the hide-and-seek game is more metaphorical than literal. That is, it might be an extended metaphor for other, less playful kinds of "hiding," such as isolating oneself from others or concealing one's true self. The poem's mysterious ending will provide further support for this interpretation.
Since most of the poem takes place in a dark shed, it relies heavily on non-visual imagery. These opening lines, for example, appeal to the sense of smell, as if "you" are sniffing around and getting your bearings in the darkness. The air has a "salty" scent, and the sacks you hide behind "smell like the seaside," presumably because they contain sand or salt. (Perhaps the "seaside" image is meant to evoke the loneliness of a deserted shore, since the shed is its own kind of isolated retreat.) Sandbags are often used as part of military fortifications, and as barriers against flooding—so if these sacks do contain sand, their presence may hint at an atmosphere of danger or unease.