When 不 occurs between two syllables like 看见 or 离开, what you have is a "potential complement".
Potential complements start with resultative complements, and insert either 得 or 不 between the syllables.
A resultative complement is made of a verb, like 看，with another syllable, such as 见 or 到, that indicates the "result" of the action of the verb. For example, 看 by itself does not mean "to see" it means to look; 见 indicates the result of looking, seeing. Looking doesn't say anything about whether you saw it or not, 见 or 到 after 看 tell you that the verb "resulted" in seeing. More examples： 听懂 listen with the result of understanding; understand via listening; 看懂 read with the result of understanding. A very clear example is 找到：找 technically just means to look for, doesn't say anything about whether you found it or not; 到 does that: seek with the result of finding: 找到。 The negative of potential complements is 没： 没看见：didn't see it; 没找到：didn't find it.
That's the background you need to understand potential complements. Potential complements tell you whether or not the action of the verb could be done. So, taking 找到：找不到 means unable to find--searching without finding; 看不见：unable to see: looked but can't see it; 听不懂： unable to understand by listening (can't understand what I'm hearing). Potential complements are most commonly seen and used in the negative: unable to achieve the action of the verb. Sometimes you'll see these in the positive form: 找得到： able to find it; 看得见： able to see it.