The nifal stem represents simple action in the passive voice (as opposed to the qal which is simple action in the active voice). video
As regards its meaning, Nifal bears some resemblance to the Greek middle voice, in being primarily reflexive of Qal, e.g. נִלְחַץ to thrust oneself (against), נִשְׁמַר to take heed to oneself, φυλάσσεσθαι, נִסְתַּר to hide oneself, נִגְאַל to redeem oneself; cf. also נַֽעֲנֶה to answer for oneself. Equally characteristic of Nifal is its frequent use to express emotions which react upon the mind; נִחַם to trouble oneself, נֶֽאֱנַח to sigh (to bemoan oneself, cf. ὀδύρεσθαι, lamentari, contristari); as well as to express actions which the subject allows to happen to himself, or to have an effect upon himself (Nifal tolerativum), e.g. דָּרַשׁ to search, to inquire, Nifal to allow oneself to be inquired of, Isa 65:1, Eze 14:3, &c.; so the Nifal of מָצָא to find, יָסַר to warn, to correct, Jer 6:8, Jer 31:18, &c.
It can also be the passive voice verb or express reciprocal or mutual action. (source)
