Ezekiel 16:4 seems to be describing practices performed on an infant or young child. But, what practice does לֹא הֻמְלַחַתְּ refer to? Do any rabbis explain the meaning?
According to Abarbanel, the salt was added in the water to strengthen the infant's body, but also (it seems) for extra hydration.
Soranus recommends that the midwife sprinkle the infant with a moderate amount of "fine and powdery salt, or natron or aphronitre." All these chemicals are mildly astringent and were recommended primarily for their ability to cut through the residue of amniotic fluid, vermix, and placenta on the newborn's skin and also to make the skin less prone to develop rashes; however, astringents would also tend to make the baby's skin dry out and flake or crack. Soranus suggests mixing the salt with honey, olive oil, or the juice of barley, fenugreek, or mallow so the granules are less likely to abrade the baby's delicate skin. The emulsion is to be washed away with warm water and the process repeated a second time.
However, according to Abraham Benisch (quoting Ben-Zeev), the root "מלח" in this case does not mean "salt", but rather "swaddle" or something similar, and this is a parallelism with the next phrase "וְהָחְתֵּל לֹא חֻתָּלְתְּ". Compare to Jer. 38:11: "בְלוֹיֵ מְלָחִים" - "worn rags". (He also compares to "מַלָח" - "sailor/seafarer", but I don't understand the comparison he makes.)
It means you weren't salted. It seems to be some sort of old tradition that somehow treating newborns with salt (externally, by rubbing, I suppose) was good for the flesh of the child.
See Rashi there. He explains that it "hardens" the flesh. Targum doesn't seem to think anything of it and "translates" it straight as salting (it's the same word in Aramaic, just slightly different form).
Mahar"i Kra explains that in other lands this was done.