The Send tab provides tools for creating outgoing transactions.
(1) Address: This is where you enter the destination address of the transaction. You can either enter a Monero address (a primary address starting with 4... or a subadress starting with 8...) or an OpenAlias address (see this page for more information about OpenAlias). Best practice is to copy and paste the address to prevent errors. Since there are malwares that may replace the address of your clipboard with an address associated with hackers, you should always verify the destination address after pasting it. Click on the Scan QR code from screen button on the title to scan a QR code from another window of your desktop. Click on the Address book button on the title to import an address from the address book of your wallet.
(2) Amount: This is how much Monero (XMR) you want to send. Click on the infinity symbol on the right side to create a transaction spending all the spendable (unlocked) balance available in your selected account. If you have enabled the option to display the balance in other currencies in Settings > Interface, your wallet will also display the amount converted in fiat currency.
(3) Add recipient: Click here if you want to send a transaction with multiple recipients (up to 16), saving on transaction fees. To remove a recipient, click on the cross symbol on the right side.
(4) Transaction priority: (requires wallet in advanced mode) This is the priority level your transaction will receive in the pool of transactions waiting to be confirmed. The higher the transaction priority, the faster your transaction will be confirmed (included in a block), but the higher you will pay in transaction fee.
It's recommended to select the default (Automatic) or Slow (x0.2 fee) option, which are likely to get your transaction into the next block (in approximately 2 minutes).
(5) Description (optional): Here you can add some information regarding your transaction for future reference. This function is for your record keeping only. The description text will be stored in your wallet file and only you will be able to see it. It will not be be sent to the receiver, nor stored on the blockchain.
Advanced options (requires wallet in advanced mode)
(6) Key images: This section allows you to export the key images from your cold wallet into a file, as well as to import this key images file into your view-only wallet. When your view-only wallet has the key images for all outputs, it can display the true balance. In order to import the key images into your view-only wallet, you must use a local node or be connected to a trusted remote node (you can set a trusted remote node in "Settings" > "Node" page). Click on the question mark icon to see step-by-step instructions.
(7) Offline transaction signing: This feature refers to the practice of keeping the private keys in a cold wallet, which is a wallet installed in an offline ("cold") device. Since the offline wallet never gets connected to the internet, its stored private keys remain secure. The cold wallet is only used to sign individual transactions, which are later sent to the Monero network using a view-only wallet. Click on the question mark icon to see step-by-step instructions.
(8) Unmixable outputs: This allows you to get rid of outputs which have strange amounts like 0.000006839355 and can't be spent without combining them with another output.
Most users will never need to use this feature.
The Address book tab lets you save the name and Monero address of contacts that you frequently transact with. This is a convenient place to copy addresses from when creating transactions.