You have initiated the process of sending coins. You came here either through the main screen or by clicking on a Bitcoin URL in your mobile browser.
First, you enter the Bitcoin address to pay to. The address will autocomplete from known entries in your address book. You can also scan QR codes with Bitcoin addresses or Bitcoin requests by pressing the QR button in the action bar.
Next, you can see how many Bitcoins you can spend at the moment. Any difference to the balance from the main screen is still unconfirmed and can thus not yet be spent.
Last, you enter the amount to pay.
You can enter a payment while being offline. It will be sent on next restart of Bitcoin Wallet. If you want a payment to be processed as fast as possible, make sure you are well connected (i.e. on a WLAN).
Note: Payments are irreversible! Always check the receiving address.