The following was a short exegetical essay for Dr. Richard E. Averbeck’s Hebrew Exegesis course at Trinity Evangelical Divinity School.
Be aware: I use the Hebrew Bible’s chapter and verse references in the following, which can be different than what one will find in English translations of Joel.
As the Dictionary of Biblical Imagery states, the winepress is “an evocative biblical image. . . .” (954). Central to the life in the ANE (Deut 15:14; 2 King 6:27) and employed with rhetorical variety, a proper understanding of the winepress’ background and usage in various Biblical themes are crucial for understanding certain portions of scripture. One of those texts is Joel 4:13, in which the entire verse involves winepress imagery.
Zondervan Dictionary of Biblical Imagery summarizes well the ancient nature of winepresses and practice of wine pressing (see also Isa 5:2; Mt 21:33; Mk 12:1):
Once the grapes ripened, they were spread in the sun for up to a week to increase their sugar content. Then the grapes were taken to the winepress where they were crushed beneath bare feet so that their precious juice might be collected. The winepress consisted of two basic parts – a gently sloping, flat floor on which the grapes could be stomped and one or more collection vats connected to the stomping floor or hewn channels (274).
Winepresses served a foundational role in Israel where drinking water was somewhat rare and water quality was a definite concern (Zondervan, 274).
In biblical-rhetorical use, the winepress often becomes a picture of fortune or lack thereof. It serves as an image of abundance (Num 18:27, 30; Deut 15:14) and blessing (Prov 3:10; Joel 2:14). A winepress harvest was cause for rejoicing (Deut 16:13-14). Therefore, a winepress’ emptiness served as a vivid example of devastation and loss (2 Kgs 6:27; cf. Job 24:11). In such cases, it emptiness often represented God’s judgment (Is 16:10; Jer 48:33; Hos 9:2; Hag 2:16; cf. Mic 6:5).
Of particular importance for Joel 4:13, the prophets often employ winepress imagery to describe the judgment and wrath of God in an extremely vivid manner. The trampling of grapes often denotes the expression of God’s wrath (Isa 63:1-6; Lam 1:15; Rev 14:19; cf. Joel 4:13).
The harm that comes to the grapes is likened to the harm that will come to those who oppose the Lord. . . . For people who stained their garments year after year in the winepress, these images were a graphic reminder of the judgment to come (Zondervan, 276).
As such, one often finds that reaping and sickle language often accompany this winepress metaphor (Joel 4:13; Rev 14:14-20). In these incidences, the juice of the crushed grapes frequently symbolizes the blood of God’s enemies (Rev 14:20; Isa 63:1-6; cf. Rev 19:13). As Crenshaw notes, “Because the juice from grapes resembled blood, the image of treading grapes was a natural one for pouring out the blood of enemies” (191).
However, the wine imagery tends to vary. For example, wine can refer to sin with which men are “intoxicated” (Rev 14:8). A harvest of grapes and vats of wine commonly represents the immense measure of human wickedness, as if iniquity is being stored up until it reaches its limit and overflows (Joel 4:13; Rev 14:14-19). Wine symbolizes God’s wrath that the wicked shall drink (Isa 63:6; Rev 14:10). Unsurprisingly, such pictures of winepress-judgment are associated with the Day of YHWH (Isa 63:4; Lam 1:12).
Concerning Joel 4:13 in particular, Joel uses grape-winepress imagery to describe God’s impending judgment against the nations. He employs a mixed metaphor of sorts (contra. Allen  who provides an explanation for harmonizing the latter two images), each image describing a distinct stage in the winemaking process. However, they all portray the same reality (note the three parallel causal כִּי clauses)—the wickedness of the nations is primed for judgment. The unharvested grapes are ripe. The winepress is filled to utter capacity; the vats are described as overflowing; evil has gone beyond the limit. YHWH says, “enough is enough!” Therefore, he commands the judgment of the nations (רְדוּ, “tread”). In sum, the winepress imagery in Joel 4:13 portrays the nations as on the verge of judgment due to their overabounding wickedness (Wolff, 80-81; Garrett, 391).