Meaning: Don’t make plans based on results/ events that may (not) happen. Don’t expect your hopes or dreams to come true.
Polish analogue: Nie chwal dnia przed zachodem słońca.
An example of use: I’m pretty sure that I’ll get a bonus this year and we’ll be able to have a nice holiday abroad. I will book it when I know for certain as I wouldn’t like to count my chickens before they hatch.