In the poem's first two lines, the speaker begins to describe a certain kind of person: one who is capable of hurting others but chooses not to do so. The speaker doesn't the specific kind of “hurt," but is likely talking about emotional, possibly romantic, injury.
The next line then sets up a paradox of sorts: this is the type of person who doesn't don’t do that very thing they seem most likely to do. The diacope here turns line 2 in particular into a tongue twister of sorts; in the short space of a single line, the speaker says the word “do” three times (four if you include line 1):
That do not do the thing they most do show,
In lines describing restraint, it's also interesting that the speaker keeps using the word “do,” which conveys action—the very opposite of restraint!
These lines are written in iambic pentameter, meaning that each line has five iambs—poetic units consisting of an unstressed syllable followed by a stressed syllable (da-DUM, da-DUM). The meter in these first two lines is mostly regular, though it's possible to read that opening "They" as a stressed beat (and note that "power" scans as a single syllable, "pow'r"):
They that have power [...]
Reading the line this way adds extra emphasis to the kind of person the speaker is describing. The second line is then perfectly iambic:
That do not do the thing they most do show
Notably, these first two lines contain very short words: the words here are all monosyllabic (consisting of one syllable). The short words, combined with close adherence to iambic pentameter, lend these lines a sense of rigidity and control. The self-restraint practiced by the type of person being described is echoed in the regular, restrained sound of the lines themselves.