After a quick look into it, you could differentiate them this way:

Both do matchmaking trying to put you into games with players of similar skill level and relatively good connection to each other.

However, in **Open**, you're able to bring your friends with you (so this way you might bring a way weaker or way stronger player with you). In **Pro** you're restricted to queue solo and the game will strictly try to find something it considers fair and with you not being able to influence who you're playing with.