This is the 6683rd most frequent Russian word.
oб
"Об" means "about" or "around" and is used as a preposition in Russian.
The word "об" is used before a pronoun ("этом"), and its usage is determined by the phonetic environment to ease pronunciation.
Я думаю об этом каждый день.
I think about this every day.
The word "об" is used before the noun "искусстве", which starts with a vowel sound for smoother spoken flow.
Мы говорили об искусстве весь вечер.
We talked about art all evening.
The word "об" takes the form "обо" before the pronoun "всём" to accommodate phonetic flow.