There are two major halachic positions about the appropriate time to daven Maariv: one is tzeis hakochavim, nightfall, when three stars appear in the sky, and the other is an earlier time, referred to on calendars as plag haminchah. The halachah is that one may follow whichever position they prefer.
However, one must be consistent and not alternate between the two positions, a situation referred to in halachah as tartei de’sasrei (an internal contradiction). If a person davens sometimes Minchah after plag haminchah, they should always wait to daven Maariv until tzeis hakochavim. (If a shul has difficulty arranging a minyan for a later time, there is room for leniency.)
On Erev Shabbos, however, a person may daven Maariv early even if they usually daven Minchah after plag haminchah because of the mitzvah of tosefes Shabbos, adding on to Shabbos—as long as on that day the person does not do a tartei de’sasrei. Nonetheless, there are those who are particular to daven Maariv after tzeis hakochavim on Erev Shabbos as well. Chabad custom is to set aside time to study Chassidus before Kabolas Shabbos, so that, usually, even Lechu Neranenah is recited after nightfall.