The gemora in Shabbos 118b says, “If only the Jewish people kept two Shabbosos according to halocha—they would immediately be redeemed!” The gemora Yerushalmi in Taanis (chapter 1, at the end of halocha 1) says, “If the Jewish people kept one Shabbos properly, the son of Dovid (i.e., Moshiach) would come.”
The seeming contradiction is resolved in Likkutei Torah (Behar 41a, from the Zohar vol. 1,) that says that there are two types of Shabbos:
1. “Lower” Shabbos: the type of rest which is experienced as a result of working during the week. This means at least two things:
a. The “rest” experience is thereby commensurate to the work during the week.
b. It is chitzonius’dik (superficial) which means that the effect is really affected. Like they say, “Easy come easy go.”
c. It’s the quality bittul (humility) called bittul hayesh.
2. “Higher” Shabbos: which is rest essential of Shabbos. Rest because Shabbos is the essence of rest and rest is the essence of Shabbos.
a. This the quality bittul called bittul b’metzius.
Image: Sefer HaMa’amorim 5723, R’ei Anochi Nosein Lifneichem HaYom, found in the Dvar Malchus of parshas R’ei, 5775.
If you want details, you’ll have to learn the ma’amorim and perhaps their sources. This is a general definition.