you snooze, you lose

You snooze, you lose.
If you are not alert you will lose an opportunity.

(The expressions states that anyone will miss out on a great opportunity if they don't remain aware or open to communication)
кто смел, тот и съел; кто успел, тот и съел

Snoozing means sleeping, or napping. The idea of this phrase is that if you let yourself get distracted from your goals (snoozing, or not paying attention), someone is going to beat you to them, and you lose.
This refers to the old story about the Tortoise racing the Hare. When the Hare got cocky and took a nap, the Tortoise passed him and won the race.

Related vocabulary:
seize the opportunity
проворонить, прозевать