Complex sentence: Only one "runlevel" is executed on bootup-run levels are "not" executed sequentially, i.e. either runlevel 2 OR 3 OR 4 is executed, "not" 2 then 3 then 4.
Split into simpler sentences: Only one "runlevel" is executed on bootup-run levels are "not" executed sequentially. Eg: either runlevel 2 OR 3 OR 4 is executed, "not" 2 then 3 then 4.