The full quote, from Act III Scene iii:
One touch of nature makes the whole world kin,
That all, with one consent, praise new-born gauds.
And give to dust that is a little gilt
More laud than gilt o’er dusted;
The present eye praises the present object.
Ulysses is trying to persuade Achilles to return to the battlefield. Kin means family, the basic meaning is that all of nature is one family.