“O Israel, your prophets have been like foxes in the waste places (ruins). You have not gone up into the gaps, nor made up the fence for the house of Israel to stand in the battle in the day of Yahweh.”
The picture is a vivid one of foxes running around in the ruins of a city. They build their dens in the ruins, and forage and scavenge, but they do nothing about the state of the city. So it is with these prophets. They have ignored the gaps in the understanding of the people, and have not built them up ready for what is coming, for they have not seen it themselves. Nor have they caused them to physically strengthen the walls of the city by their warnings. Instead of ‘rebuilding the walls and filling in the gaps', by preparing the people for the coming ‘day of Yahweh' about to fall on them, they are like foxes who make comfortable holes for themselves in the ruins and scurry around building nothing, scavenging for what they can find, making false promises that will not be fulfilled. They are nothing but parasites.
Note the phrase ‘the day of Yahweh'. It refers to any period in history where God manifests His judgments. God is longsuffering and gives man much leeway, but there comes a time again and again when man's sins come in on himself and devastating consequences result. And each such ‘day of Yahweh' leads on to the next, until the final great ‘day of Yahweh' when He brings in His final judgments.
(The alternation between ‘they' and ‘you' in these verses makes it uncertain when it is the people who are being addressed, and when the prophets, but it makes no difference to the sense. If ‘you' is seen as applying to the people it simply incorporates them into the sins of the prophets).