Therein, it tells about the action, thus (it is) the verb, the word
for the action.
Notes:
1) "tattha" (therein) refers to the preceeding verse.
2) "kiriya.m akkhaayati" is understood here as an active sentence
with 'kiriya.m' as the object (kamma) of the transitive verb
'akkhaayati'. The regular active form is 'akkhaati' (aa + khaa + a +
ti) and the passive form is 'akkhaayati' (aa + khaa + a + ya + ti).
Cone (DOP I 5f) gives 'akkhaayati' as both a passive and as an
alternative form for 'akkhaati'. How then does one account for
the -ya- in the active form? One solution could be to take -y- as an
inserted letter (aagama) to prevent the following vikara.na -a- from
disappearing but this seems unlikely. At Sadd II 326 both verbal roots
/khaa/ and /khyaa/ are given together in the sense of pakathana
(pakathana.m aacikkhana.m desana.m vaa). The form 'akkhaayati' is
given there in the passive only. I think 'akkhaayati' can also be a
causative (aa + khaa + ?a + .naya + ti) and perhaps the impersonal or
stative has the same form too. It's an ambjguos verb.
3) "aakhyaata.m" -- the (finite) verb -- is an agent-noun in the
neuter as far as I can tell. It can be resolved into the prefix 'aa' +
the root /khyaa/ + the past participle suffix 'ta' + the vibhatti
suffix 'si' for the nom. sing. Sd 1144 (Sadd III 850) explains the use
of 'ta' to denote the agent as in 'buddho'. For other etymologies we
have at Sadd II 326,23: "kiriya.m aakhyaati kathetiiti aakhyaata.m".
And, at Ruup p. 257 (Be), there is "kriya.m aacikkhatiiti aakhyaata.m
kriyaapada.m".