The word kilter is very nearly a fossil word, only very rarely being used outside of the phrases out of kilter and off kilter. However, it means exactly what these phrases would suggest:
Kilter: Regular order or proper condition.
This word is so archaic that the on-line Oxford Dictionary defines it only in the 'out of kilter' construction, and even Webster1913 prefers the older form, 'Kelter'. However you spell it, it appeared mysteriously in England in the early 1600s, perhaps from the same root as kilt, the Middle English verb kilten, meaning "to tuck up". However the only support for this theory is that it fits better than anything else.
Off kilter and out of kilter both mean out of sorts, unbalanced, wonky, eccentric, or otherwise not as wanted, but not catastrophically so. Most American and UK speakers of English will still recognize these usages.
The positive uses of kilter are less common, and may surprise your friends and relations. However, the phrases in kilter, in good kilter, in high kelter, and to get into kilter are all proper English, if slightly archaic, and you are encouraged to use them.