BNB is the native currency of Binance, one of the most popular cryptocurrency exchanges. BNB can both be used as a currency for transactions and on the Binance platform for trading fee discounts.
From ZenGo, you can send BNB to anyone by entering their BNB address into the address field, but custodial services (e.g. exchanges) may also require you to enter a memo with the address. This is because these services have many users associated under the same BNB address, so the memo allows the exchange to distinguish the recipient from all the other users of the service. You can think of a memo like the recipient's unique username.
If the recipient requires a memo to receive BNB it will likely be indicated somewhere near their BNB address in the service they're using.
To receive BNB in ZenGo, you DO NOT need a memo. Since ZenGo is a non-custodial wallet, your BNB address is unique to you and is not shared with anyone else, so all you need to receive BNB in ZenGo is your BNB address.