so I want to know when I give out this public key to receive bitcoin then after the expiring date, can I still use the address again or I will go and get another one?
The address doesn’t expire so does your coins on it too do not expire. You can reuse the address as many times as like but just like other HD wallets the electrum gives you the opportunity to always have 20 unused address as receiving addresses.
The expiration is used by electrum wallet to know that the address cannot be used for another invoice or payment request if the first request it is used for hasn’t expired. But it gets expired and no bitcoin is received then the wallet can use it again for another request.
You can also use it to set a reminder that you’re suppose to receive a payment within that period of time.