A Cryptocurrency wallet is a piece of information that aides Cryptocurrency transactions from one individual (sender),  to another (receiver/buyer).

In order to send or receive Cryptocurrency coins, you need to use a Cryptocurrency "wallet" 

Unlike a wallet in the real world — that actually holds your cash (and debit cards & credit cards), a cryptocurrency wallet doesn't actually "hold" your Cryptocurrency coins.

Instead,  a cryptocurrency wallet is a hard device or software program that interacts with Cryptocurrency coins' blockchain by providing the necessary information needed to create Cryptocurrency transactions. 

One piece of information is called a "wallet address" or simply "an address"

Cryptocurrency transactions can be described as the transfer of cryptocurrency coin from one address to another address. 

Cryptocurrency wallets store "keys", unlike physical wallets that that store physical cash, — and one of those keys is known as a "private key", which is what allows one to send or move Cryptocurrency tokens or coins from one wallet address to another wallet address. 

When dealing with Crypto wallets, you'll always come across terms like, "private key, public key, address,  and seed phrase."

To send and receive money online, you usually need an "account number" and a "password". In the Cryptocurrency world, these are called "public key" and a "private key" respectively.

Meaning;

Account number = Public Key = Address, 

and,

Password = Private Key.

Cryptocurrencies like Bitcoin uses a two-key system, in which Public Key allows you to RECEIVE bitcoins, while the Private Key is needed to SEND bitcoins.

The private key is a unique secret number that only you should know. It is used to sign transactions.  They are known as digital signatures.

The public key is a crazy long number, so a shortened version of the public key is used instead,  making it easier to read.

What happens if you lose your private key?

Losing your private key is equivalent to losing your cash. It's therefore impossible to figure out your private key from either your public kwy or address. 

Assume, you lose your cash in a crowded area, then after one hour, you go look for it. Will you find it?

That's what it means to lose a private key.