The headword as transmitted [but see below] is nominative singular of the masculine noun probolos, for which cf. pi 2348, pi 2351, pi 2352, pi 2353, and see generally LSJ s.v. The gloss (which includes a neuter noun for which see already under epsilon 1504 and kappa 2338) indicates that it is to be construed impersonally.
[1] Likewise or similarly in other lexica; references at Photius pi1195, where Theodoridis obelizes the transmitted headword. In her critical apparatus Adler notes that Kuster suggested comparing this passage to pi 2497, pro/lobos (bird's crop); some Suda mss appear to have confused the two headwords (cf. pi 2497 n.1); and indeed the present gloss is far better suited to prolobos than to any attested sense of probolos. See now Theodoridis on the matter.