"That plate is round."
お o- is an exalted prefix. It shows respect for the individual or object to whom it pertains. ご go- plays a similar role, being attached to a separate group of words as explained below. Both お o- and ご go- can be written as 御. お o- can be applied to nouns, na-adjectives, i-adjectives, and verbs.
Because お o is exalted, it is not used on or about the speaker. For example,
お元気げんきですか。 O-genki desu ka?
is the formal way of asking "How are you." The normal response is
元気げんきです。 Genki desu.
In the first example, person A is exalting person B by placing the exalted prefix お o- in front of the word 元気げんき genki, which refers to person B's health. In the response, person B omits the お o-prefix because he does not wish to exalt himself.
Note that お o- is usually only applied to words of Japanese origin, and most Sino-Japanese words take the honorific prefix 御ご go- instead, as in 御主人ごしゅじん master/husband. (元気げんき genki is a notable exception.)