Sobre is the forty seventh most common word in the Spanish Language.
This lively two-syllable is both a masculine noun and a preposition. My Collins Spanish -English dictionary gives the following key meanings of the word.
Sobre as a masculine noun means an “envelope”
Sobre as a preposition means “on”.
It is clear though that most of the use of sobre will be with its meaning of “on”.
Here is an example of each of the meanings.
¿Dónde está el sobre? – where is the envelope? Sobre, therefore, is being used as as a masculine noun.
Sobre la mesa – on the table In this case, sobre is being used as a preposition.
And here is a sentence, putting both of the previous examples of both the noun and the preposition into action. Such fun!
El sobre está sobre la mesa – The envelope is on the table
To conclude – sobre is the 47th most common word in the Spanish language. It doubles up as both a masculine noun and a preposition. Such are the varied lives led by these high frequency Spanish words!