Both of these adjectives are formed from the verb tolerate, which means “allow, permit, or endure “—i.e. “not resist, not oppose.”
Intolerable (in-toler-able) therefore describes something which cannot or will not be allowed or endured—anything, from personalities to physical forces:
I find television intolerable; it is a waste of time at best, stupefying at worst.
Rockets are designed to withstand intolerable amounts of stress.
Intolerant (in-toler-ant) describes a person or attitude that does not allow or not accept something:
I am intolerant of television, especially because of the commercials.
The bridge made of concrete would be intolerant of earthquakes.
I love all kinds of people and I don’t understand intolerant people, like racists.
The last sentence above shows a common idiomatic use of the word intolerant—“prejudiced against people of a different race, religion, or sexual orientation than one’s own”—in other words, “intolerant of people different from oneself.” We especially often say racial intolerance for racism. Frankly, this use of the word “intolerant” because it implies that racial differences are unpleasant, rather than wonderful and enriching.