The impersonal verb uses a dummy subject (il = it), as in
The il or it do not refer to anything in particular here. They are dummy forms. The meaning is Leaving nothing to chance is important. The most common impersonal form is il y a = there is, there are. Note its other tenses (il y avait, il y a eu, il y aura, etc.).
The ninth most common verb, falloir, only exists an an impersonal. There is no form for je or tu.
The most common impersonal, apart from il faut, is il s'agit, which combines an impersonal with a pronominal, but is found in the dictionary under V IMPERS. Il s'agit basically means it's a question of , it's about, but the ways this comes out in English vary very widely. This expression is found in most informative and argumentative texts of a certain length, so it's worth while spending a little time on it. Check it out in your dictionary, copy out a few of the common phrases and keep reviewing them. Here are a few common ways it's used.
With other verbs, the meaning of the impersonal form may be slightly different from that of the standard form, though usually not as different as in the cases of pronominal verbs. If you find an impersonal form in a text, check the impersonal form section of the verb entry in the dictionary. This will make it quicker and easier to find idioms or special meanings. Check out the following in your dictionary.
convenir - to suit
rester - to stay
se trouver - to be