FirstTo begin רוח has several meanings. The main two are wind and spirit. Second of all, what does it mean for a a creature to be alive? This may seem like an abstract, unrelated question, but it is the key to understanding what is meant by רוח in this passage. To answer this question, we look at Genesis 2:7 where Man first comes to life which states "וַיִּיצֶר֩ יְהוָ֨ה אֱלֹהִ֜ים אֶת־הָֽאָדָ֗ם עָפָר֙ מִן־הָ֣אֲדָמָ֔ה וַיִּפַּ֥ח בְּאַפָּ֖יו נִשְׁמַ֣ת חַיִּ֑ים וַֽיְהִ֥י הָֽאָדָ֖ם לְנֶ֥פֶשׁ חַיָּֽה׃" "And the Master G-d created the Man out of dust from the ground. He blew into his nose the breath of life and Man became a living being" (Genesis 2:7). This passage makes it clear that something becomes a living being when the "breath of life" is breathed into it. This "breath of life" is what makes something living. Thus, it would be logical to conclude that in Ezekiel 37, as the bodies return to life when the רוח is placed inside of them, this רוח is the same "breath of life." As for what this "breath of life" means, it is most likely the soul. Thus the רוח is most likely the souls of the people who return to life in the passage.