A professional — someone who does an activity as their job and has high skill or expertise.
彼はサッカーのプロだ。
He is a professional soccer player.
プロの仕事は仕上がりが違う。
A professional's work looks different (i.e., is of higher quality).
彼女はプロ並みの腕前だ。
She has skills comparable to a professional.
この仕事はプロに頼んだほうがいい。
It's better to ask a professional to do this job.
Abbreviation of 'program' (プログラム); can mean a program or schedule in certain contexts (usage is limited and context-dependent).
放送業界では『プロ』がプログラムの略で使われることがある。
In the broadcasting industry, 'プロ' is sometimes used as an abbreviation for 'program'.
Abbreviation of 'production' (プロダクション); can refer to a production company or talent agency.
彼は大手プロに所属している。
He belongs to a major production company / talent agency.
あの新人は有名なプロからデビューした。
That newcomer debuted under a well-known production company.
Abbreviation of 'proletarian' (proletariat); used in political or historical contexts (somewhat dated).
その時代の文献にはプロ（プロレタリア）を擁護する記述が多い。
Many writings from that era contain passages supporting the proletariat.
Abbreviation of 'propaganda' (プロパガンダ); can refer to propaganda or propaganda efforts (context-dependent).
その新聞は一方的な主張でプロ（プロパガンダ）だと批判された。
That newspaper was criticized as propaganda for its one-sided claims.