"Erratic weather" means weather that's strange for the season, and that changes a lot from day to day. People usually use this phrase to talk about weather that's warm on one day and then much cooler the next day.
The word "erratic" can also be used to describe some other things, like a person's behavior:
His behavior has been awfully erratic lately.
Like erratic weather, "erratic behavior" is when a person acts differently from day to day. An "erratic" person changes their personality suddenly.