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.
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.