These are subject proclitic pronouns.
These do vary considerably across the Gallo-Romance branches, and it is hard to generalise. What is relatively well known is the correspondence between the clitics and their Latin origins. Taking Paduan / padovese as an example:
1S: null proclitic; enclitic = -i, from Latin
EGO > ieo
2S: proclitic = te; enclitic = -to, from Latin
TV > tu, te
3S-m: proclitic = el; enclitic = -lo from Latin
ILLUS / ILLOS
3S-f: proclitic = la, enclitic = -la from Latin
1P: null proclitic; enclitic = -i, generalised from 1S
2P: null proclitic; enclitic = -u, from Latin
3S-m: proclitic = i; enclitic = -li from Latin
3S-f: proclitic = le, enclitic = -le from Latin
15th century Renaissance Venetian was a pro-drop language, although it retained the possibility to have a subject pronoun in embedded clauses too, in a way that e.g. Modern Italian doesn't need to:
Venetian: El me par che 'l sarave cossa giusta.
Gloss: It to-me seems that it be-COND thing right-FEM
English translation: It seems to me that it would be the right thing.
cf. modern Italian: Mi sembra che sarebbe la cosa giusta.
Thus it had a complete set of subject clitics and an expletive / impersonal pronoun.
1S: a / e
2S: te / ti
1P: a / e
2P: a / e
We see that the 1st person singular has spread to 1st person plural and 2nd person plural. There is a kind of syntactico-semantic split there, where 2nd person singular as well as 3rd person singular and plural clitics are retained as separate.
Thus, the subject pronouns have never been a substitute for verbal morphology in Venetian, unlike French, where the endings eroded faster than the subject pronouns (although French still produced a class of disjunctive pronouns).
Venetian of the 17th century on the other hand started exhibiting these subject proclitic pronouns:
Venetian: Perchè no la pole.
Gloss: Because not she can
English: Because she cannot
cf. modern Italian: Perché (ella/essa/lei) non può.
Hence between the 15th and 17th centuries, the subject clitic had been reanalysed. Thus, the loss of agreement in Venetian finds its parallel in the evolution of early modern French, where pro-drop is functionally extinct in all but the most specific contexts (under inversion, where the subject is explicitly given with certain verbs of motion, e.g. Quand viendra Jean ?). As summarised in Poletto (1995):
Both SVe [17th century Venetian] and French have developed into systems in which Agr is not a possible pro-drop licenser. This is the reason why they both have maintained subject clitics.
In standard French subject clitics have remained true subjects. [...] On the contrary, in all Northern Italian dialects subject clitics have been reinterpreted as a possible candidate to substitute agreement in various syntactic mechansims.