What does “akin to” mean in etymologies in dictionary entries?

When they indicate that a word comes from a language, it indicates a direct, known relationship between the two words. When they switch to saying "akin to," the entry indicates that there is likely an indirect relationship between the word in question and the word it is akin to.

A quick lookup indicates that English originated from a West German dialect, which I'm sure relates to Old High German in some manner. Going to the example in salt, the entry indicates that there is the Old English word and the Old High German word. However, there's at least one language in between those two that the etymology flows through. Since the Old High German is only akin to the Old English, all the ancestor words from the Old High German are also akin to the present-day word.

In short, there's a missing link in the etymological chain. The term "akin to" recognizes that fact.

From the NOAD:

akin /əˈkɪn/
adjective
of similar character: something akin to gratitude overwhelmed her | genius and madness are akin.
• related by blood.
ORIGIN middle 16th century: contracted form of of kin.

In the case of words, it means there is a similarity between the words, or they have a common (or shared) root.

Reading what the NOAD says about salt, I find:

ORIGIN
Old English sealt (noun), sealtan (verb), of Germanic origin; related to Dutch zout and German Salz (nouns), from an Indo-European root shared by Latin sal, Greek hals 'salt'.

The New Oxford American Dictionary uses related to where the Merriam-Webster Online Dictionary uses akin to.

That word is so close to another word in another language they are practically family. They may share a common ancestor word or just sound so alike that a relationship is imagined.

Akin simply means similar to.